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.

nuts_bolts.h

@@ -2,7 +2,7 @@
   nuts_bolts.h - Header file for shared definitions, variables, and functions
   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
@@ -30,15 +30,23 @@
 #define false 0
 #define true 1
 
+// Axis array index values. Must start with 0 and be continuous.
 #define N_AXIS 3 // Number of axes
-#define X_AXIS 0 // Axis indexing value. Must start with 0 and be continuous.
+#define X_AXIS 0 // Axis indexing value. 
 #define Y_AXIS 1
 #define Z_AXIS 2
 // #define A_AXIS 3
 
+// CoreXY motor assignments. DO NOT ALTER.
+// NOTE: If the A and B motor axis bindings are changed, this effects the CoreXY equations.
+#ifdef COREXY
+ #define A_MOTOR X_AXIS // Must be X_AXIS
+ #define B_MOTOR Y_AXIS // Must be Y_AXIS
+#endif
+
+// Conversions
 #define MM_PER_INCH (25.40)
 #define INCH_PER_MM (0.0393701)
-
 #define TICKS_PER_MICROSECOND (F_CPU/1000000)
 
 // Useful macros