Lot of refactoring for the future. CoreXY support.

- Rudimentary CoreXY kinematics support. Didn’t test, but homing and
feed holds should work. See config.h. Please report successes and
issues as we find bugs.

- G40 (disable cutter comp) is now “supported”. Meaning that Grbl will
no longer issue an error when typically sent in g-code program header.

- Refactored coolant and spindle state setting into separate functions
for future features.

- Configuration option for fixing homing behavior when there are two
limit switches on the same axis sharing an input pin.

- Created a new “grbl.h” that will eventually be used as the main
include file for Grbl. Also will help simply uploading through the
Arduino IDE

- Separated out the alarms execution flags from the realtime (used be
called runtime) execution flag variable. Now reports exactly what
caused the alarm. Expandable for new alarms later on.

- Refactored the homing cycle to support CoreXY.

- Applied @EliteEng updates to Mega2560 support. Some pins were
reconfigured.

- Created a central step to position and vice versa function. Needed
for non-traditional cartesian machines. Should make it easier later.

- Removed the new CPU map for the Uno. No longer going to used. There
will be only one configuration to keep things uniform.

main.c

@@ -2,7 +2,7 @@
   main.c - An embedded CNC Controller with rs274/ngc (g-code) support
   Part of Grbl v0.9
   
-  Copyright (c) 2012-2014 Sungeun K. Jeon
+  Copyright (c) 2012-2015 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
@@ -89,7 +89,8 @@ int main(void)
 
     // Reset system variables.
     sys.abort = false;
-    sys.execute = 0;
+    sys.rt_exec_state = 0;
+    sys.rt_exec_alarm = 0;
     if (bit_istrue(settings.flags,BITFLAG_AUTO_START)) { sys.auto_start = true; }
     else { sys.auto_start = false; }