G54 work coordinate support (w/ G10,G92.1). Re-factored g-code parser with error checking. Minor compiler compatibility changes.

- G54 work coordinate system support. Up to 6 work coordinate systems
(G54-G59) available as a compile-time option.

- G10 command added to set work coordinate offsets from machine
position.

- G92/G92.1 position offsets and cancellation support. Properly follows
NIST standard rules with other systems.

- G53 absolute override now works correctly with new coordinate systems.

- Revamped g-code parser with robust error checking. Providing user
feedback with bad commands. Follows NIST standards.

- Planner module slightly changed to only expected position movements
in terms of machine coordinates only. This was to simplify coordinate
system handling, which is done solely by the g-code parser.

- Upon grbl system abort, machine position and work positions are
retained, while G92 offsets are reset per NIST standards.

- Compiler compatibility update for _delay_us().

- Updated README.

readme.textile

@@ -14,9 +14,10 @@ Grbl includes full acceleration management with look ahead. That means the contr
   - New run-time command control: Feed hold (pause), Cycle start (resume), Reset (abort), Status reporting
   - Controlled feed hold with deceleration to ensure no skipped steps and loss of location.
   - After feed hold, cycle accelerations are re-planned and may be resumed.
-  - Work offset(G92) and machine coordinate system support.
-  - Program stop(M0,M1*,M2,M30) initial support. Pallet shuttle not supported.
-  - System reset re-initializes grbl without resetting the Arduino and retains machine/home position.
+  - Re-factored g-code parser with robust error-checking.
+  - Work coordinate system (G54), offsets(G92), and machine coordinate system support. G10 work coordinate settings support. (Up to 6 work coordinate systems(G54-G59) available as a compile-time option.)
+  - Program stop(M0,M1*,M2,M30) initial support. Optional stop to do.
+  - System reset re-initializes grbl without resetting the Arduino and retains machine/home position and work coordinates.
   - Restructured planner and stepper modules to become independent and ready for future features.
   - Planned features: Jog mode, status reporting, runtime settings such as toggling block delete, XON/XOFF flow control.
   - Reduce serial read buffer to 128 characters and increased write buffer to 64 characters.