Refactoring and lots of bug fixes. Updated homing cycle.

WARNING: There are still some bugs to be worked out. Please use caution
if you test this firmware.

- Feed holds work much better, but there are still some failure
conditions that need to be worked out. This is the being worked on
currently and a fix is planned to be pushed next.

- Homing cycle refactoring: Slight adjustment of the homing cycle to
allow for limit pins to be shared by different axes, as long as the
shared limit pins are not homed on the same cycle. Also, removed the
LOCATE_CYCLE portion of the homing cycle configuration. It was
redundant.

- Limit pin sharing: (See above). To clear up one or two limit pins for
other IO, limit pins can now be shared. For example, the Z-limit can be
shared with either X or Y limit pins, because it’s on a separate homing
cycle. Hard limit will still work exactly as before.

- Spindle pin output fixed. The pins weren’t getting initialized
correctly.

- Fixed a cycle issue where streaming was working almost like a single
block mode. This was caused by a problem with the spindle_run() and
coolant_run() commands and issuing an unintended planner buffer sync.

- Refactored the cycle_start, feed_hold, and other runtime routines
into the runtime command module, where they should be handled here
only. These were redundant.

- Moved some function calls around into more appropriate source code
modules.

- Fixed the reporting of spindle state.

README.md

@@ -15,17 +15,20 @@ Grbl includes full acceleration management with look ahead. That means the contr
 
 ##Changelog for v0.9 from v0.8
   - **ALPHA status: Under heavy development.**
-  - New stepper algorithm:  Based on an inverse time algorithm, but modified to ensure steps are executed exactly. This algorithm performs a constant timer tick and has a hard limit of 30kHz maximum step frequency. It is also highly tuneable and should be very easy to port to other microcontroller architectures. Overall, a much better, smoother stepper algorithm with the capability of very high speeds.
-  - Planner optimizations: Planning computations improved four-fold or more. Changes include streaming optimizations by ignoring already optimized blocks and removing redundant variables and computations and offloading them to the stepper algorithm on an ad-hoc basis.
+  - New stepper algorithm:  Complete overhaul of the handling of the stepper driver to simplify and reduce task time per ISR tick. Much smoother operation with the new Adaptive Multi-Axis Step Smoothing (AMASS) algorithm which does what its name implies. Users should audibly hear significant differences in how their machines move and see improved overall performance!
+  - Planner optimizations: Planning computations improved four-fold or more by optimizing end-to-end operations. Changes include streaming optimizations by ignoring already optimized blocks and removing redundant variables and computations and offloading them to the stepper algorithm to compute on an ad-hoc basis.
   - Planner stability: Previous Grbl planners have all had a corruption issue in rare circumstances that becomes particularly problematic at high step frequencies and when jogging. The new planner is robust and incorruptible, meaning that we can fearlessly drive Grbl to it's highest limits. Combined with the new stepper algorithm and planner optimizations, this means 5x to 10x performance increases in our testing! This is all achieved through the introduction of an intermediary step segment buffer that "checks-out" steps from the planner buffer in real-time. 
   - Acceleration independence: Each axes may be defined with different acceleration parameters and Grbl will automagically calculate the maximum acceleration through a path depending on the direction traveled. This is very useful for machine that have very different axes properties, like the ShapeOko z-axis.
   - Maximum velocity independence: As with acceleration, the maximum velocity of individual axes may be defined. All seek/rapids motions will move at these maximum rates, but never exceed any one axes. So, when two or more axes move, the limiting axis will move at its maximum rate, while the other axes are scaled down.
-  - Significantly improved arc performance: Arcs are now defined in terms of chordal tolerance, rather than segment length. Chordal tolerance will automatically scale all arc line segments depending on arc radius, such that the error does not exceed the tolerance value (default: 0.005 mm.) So, for larger radii arcs, Grbl can move faster through them, because the segments are always longer and the planner has more distance to plan with.
+  - Significantly improved arc performance: Arcs are now defined in terms of chordal tolerance, rather than segment length. Chordal tolerance will automatically scale all arc line segments depending on arc radius, such that the error does not exceed the tolerance value (default: 0.002 mm.) So, for larger radii arcs, Grbl can move faster through them, because the segments are always longer and the planner has more distance to plan with.
   - Soft limits: Checks if any motion command exceeds workspace limits. Alarms out when found. Another safety feature, but, unlike hard limits, position does not get lost, as it forces a feed hold before erroring out.
-  - Pin mapping: In an effort for Grbl to be compatible with other AVR architectures, such as the 1280 or 2560, a new pin_map.h configuration file has been created to allow Grbl to be compiled for them. This is currently user supported, so your mileage may vary. If you run across a bug, please let us know or better send us a fix! Thanks in advance!
+  - CPU pin mapping: In an effort for Grbl to be compatible with other AVR architectures, such as the 1280 or 2560, a new cpu_map.h pin configuration file has been created to allow Grbl to be compiled for them. This is currently user supported, so your mileage may vary. If you run across a bug, please let us know or better send us a fix! Thanks in advance!
   - New Grbl SIMULATOR by @jgeisler: A completely independent wrapper of the Grbl main source code that may be compiled as an executable on a computer. No Arduino required. Simply simulates the responses of Grbl as if it was on an Arduino. May be used for many things: checking out how Grbl works, pre-process moves for GUI graphics, debugging of new features, etc. Much left to do, but potentially very powerful, as the dummy AVR variables can be written to output anything you need. 
   - Homing routine updated: Sets workspace volume in all negative space regardless of limit switch position. Common on pro CNCs. Now tied directly into the main planner and stepper modules to reduce flash space and allow maximum speeds during seeking.
-  - Feedrate overrides: In the works, but planner has begun to be re-factored for this feature.
-  - Jogging controls: Methodology needs to be to figured out first. Could be dropped due to flash space concerns. Last item on the agenda.
+  - Combined limit pins capability: Limit switches can be combined to share the same pins to free up precious I/O pins for other purposes. When combined, users must adjust the homing cycle mask in config.h to not home the axes on a shared pin at the same time. 
+  - Variable spindle speed output: Available only as a compile-time option through the config.h file. Enables PWM output for 'S' g-code commands. Enabling this feature will swap the Z-limit D11 pin and spindle enable D12 pin to access the hardware PWM on pin D12. The Z-limit pin, now on D12, should work just as it did before.
+  - Increased serial baud rate: Default serial baudrate is now 115200, because the new planner and stepper allows much higher processing and execution speeds that 9600 baud was just not cutting it.
+  - Feedrate overrides: (Slated for v1.0 release) The framework to enable feedrate overrides is in-place with v0.9, but the minor details has not yet been installed.
+  - Jogging controls: (Slated for v1.0 release) Methodology needs to be to figured out first. Could be dropped due to flash space concerns.
   
 _The project was initially inspired by the Arduino GCode Interpreter by Mike Ellery_