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.

stepper.h

@@ -2,7 +2,7 @@
   stepper.h - stepper motor driver: executes motion plans of planner.c using the stepper motors
   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
@@ -34,7 +34,7 @@
 // Initialize and setup the stepper motor subsystem
 void stepper_init();
 
-// Enable steppers, but cycle does not start unless called by motion control or runtime command.
+// Enable steppers, but cycle does not start unless called by motion control or realtime command.
 void st_wake_up();
 
 // Immediately disables steppers
@@ -46,13 +46,13 @@ void st_generate_step_dir_invert_masks();
 // Reset the stepper subsystem variables       
 void st_reset();
              
-// Reloads step segment buffer. Called continuously by runtime execution system.
+// Reloads step segment buffer. Called continuously by realtime execution system.
 void st_prep_buffer();
 
 // Called by planner_recalculate() when the executing block is updated by the new plan.
 void st_update_plan_block_parameters();
 
-// Called by runtime status reporting if realtime rate reporting is enabled in config.h.
+// Called by realtime status reporting if realtime rate reporting is enabled in config.h.
 #ifdef REPORT_REALTIME_RATE
 float st_get_realtime_rate();
 #endif