Tweaks and minor bug fixes. Added purge buffer command.

- Added a purge buffer (and lock) command. This is an advanced option
to clear any queued blocks in the buffer in the event of system
position being lost or homed. These queued blocks will likely not move
correctly if not purged. In typical use, the purging command releases
the homing axes lock in case a user need to move the axes off their
hard limit switches, but position is not guaranteed. Homing is advised
immediately after.

- Created a system-wide sync current position function. Cleans up some
of the repetitive tasks in various places in the code that do the same
thing.

- Removed the clear all switches command '$S'. Not really needed and
helped clean up a sync call.

- Other minor tweaks. Readme updated slightly..

readme.textile

@@ -21,13 +21,13 @@ Grbl includes full acceleration management with look ahead. That means the contr
   - Re-factored g-code parser with robust error-checking.
   - 6 work coordinate systems (G54-G59), offsets(G92), and machine coordinate system support. Work coordinate systems are stored in EEPROM and persistent.
   - G10 L2 and L20 work coordinate settings support. L2 sets one or more axes values. L20 sets the current machine position to the specified work origin.
-  - Program stop(M0,M1,M2,M30) initial support.
+  - G28.1 and G30.1 set home position support. These set the internal EEPROM parameter values to the current machine position. (G28 and G30 no longer perform homing cycle, '$H' does. They move to these stored positions.)
+  - Program stop(M0,M1,M2,M30) support.
   - Coolant control(M7*,M8,M9) support. (M7 is a compile-time option).
   - System reset re-initializes grbl without resetting the Arduino and retains machine/home position and work coordinates.
   - Settings overhauled and dozens of new settings and internal commands are now available, when most were compile-time only.
   - New startup line setting. Allows users to store a custom g-code block into Grbl's startup routine. Executes immediately upon startup or reset. May be used to set g-code defaults like G20.
   - Misc bug fixes and removed deprecated acceleration enabled code.  
-  - Planned features: Axis acceleration and max speed individual settings, full-featured status reporting.
   - Advanced compile-time options: XON/XOFF flow control (limited support), direction and step pulse time delay, and up to 5 startup lines.