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.

examples/Grbl/Grbl.ino

@@ -0,0 +1,22 @@
+#include <config.h>
+#include <coolant_control.h>
+#include <cpu_map.h>
+#include <defaults.h>
+#include <eeprom.h>
+#include <gcode.h>
+#include <grbl.h>
+#include <limits.h>
+#include <motion_control.h>
+#include <nuts_bolts.h>
+#include <planner.h>
+#include <print.h>
+#include <probe.h>
+#include <protocol.h>
+#include <report.h>
+#include <serial.h>
+#include <settings.h>
+#include <spindle_control.h>
+#include <stepper.h>
+#include <system.h>
+
+#include <src/main.h>