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.

gcode.h

@@ -3,7 +3,8 @@
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
-
+  Copyright (c) 2011-2012 Sungeun K. Jeon
+  
   Grbl is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or