Program stop support (M0,M1*,M2,M30*), proper position retainment upon reset, misc minor updates.

- Program stop support (M0,M1*,M2,M30*). *Optional stop to be done.
*Pallet shuttle not supported.

- Work position is set equal to machine position upon reset, as
according to NIST RS274-NGC guidelines. G92 is disabled.

- Renamed mc_set_current_position() to mc_set_coordinate_offset().

- Fixed bug in plan_synchronize(). Would exit right before last step is
finished and caused issues with program stops. Now fixed.

- Spindle now stops upon a run-time abort command.

- Updated readme and misc upkeeping.

motion_control.h

@@ -3,7 +3,7 @@
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
-  Copyright (c) 2011 Sungeun K. Jeon
+  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
@@ -27,7 +27,7 @@
 
 // NOTE: Although the following function structurally belongs in this module, there is nothing to do but
 // to forward the request to the planner.
-#define mc_set_current_position(x, y, z) plan_set_current_position(x, y, z) 
+#define mc_set_coordinate_offset(x, y, z) plan_set_coordinate_offset(x, y, z) 
 
 // Execute linear motion in absolute millimeter coordinates. Feed rate given in millimeters/second
 // unless invert_feed_rate is true. Then the feed_rate means that the motion should be completed in