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.

limits.c

@@ -24,14 +24,8 @@
     Copyright (c) 2012 Sungeun K. Jeon
 */  
   
-#include "system.h"
-#include "settings.h"
-#include "protocol.h"
-#include "planner.h"
-#include "stepper.h"
-#include "motion_control.h"
-#include "limits.h"
-#include "report.h"
+#include "grbl.h"
+
 
 // Homing axis search distance multiplier. Computed by this value times the axis max travel.
 #define HOMING_AXIS_SEARCH_SCALAR  1.5 // Must be > 1 to ensure limit switch will be engaged.