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.

coolant_control.c

@@ -18,10 +18,7 @@
   along with Grbl.  If not, see <http://www.gnu.org/licenses/>.
 */  
 
-#include "system.h"
-#include "coolant_control.h"
-#include "protocol.h"
-#include "gcode.h"
+#include "grbl.h"
 
 
 void coolant_init()