Bug fix for certain motions. Re-org of includes.

- Critical bug fix for diagonal motions that continue on the same
direction or return in the exact opposite direction. This issue could
cause Grbl to crash intermittently due to a numerical round-off error.
Grbl versions prior to v0.9g shouldn’t have this issue.

- Reorganized all of the includes used by Grbl. Centralized it into a
single “grbl.h” include. This will help simplify the compiling and
uploading process through the Arduino IDE.

- Added an example .INO file for users to simply open and run when
compiling and uploading through the IDE. More to come later.

gcode.c

@@ -24,15 +24,7 @@
     Copyright (c) 2011-2012 Sungeun K. Jeon
 */  
 
-#include "system.h"
-#include "settings.h"
-#include "protocol.h"
-#include "gcode.h"
-#include "motion_control.h"
-#include "spindle_control.h"
-#include "coolant_control.h"
-#include "probe.h"
-#include "report.h"
+#include "grbl.h"
 
 // NOTE: Max line number is defined by the g-code standard to be 99999. It seems to be an
 // arbitrary value, and some GUIs may require more. So we increased it based on a max safe