Third time's a charm! No more deceleration issues! Updated grbl version and settings. General cleanup.

- Fleshed out the original idea to completely remove the long slope at
the end of deceleration issue. This third time should absolutely
eliminate it.
- Changed the acceleration setting to kept as mm/min^2 internally,
since this was creating unneccessary additional computation in the
planner. Human readable value kept at mm/sec^2.
- Updated grbl version 0.7d and settings version to 4. NOTE: Please
check settings after update. These may have changed, but shouldn't.
- Before updating the new features (pause, e-stop, federate override,
etc), the edge branch will soon be merged with the master, barring any
immediate issues that people may have, and the edge branch will be the
testing ground for the new grbl version 0.8.

motion_control.c

@@ -42,12 +42,6 @@ void mc_dwell(double seconds)
    }
 }
 
-// Execute an arc in offset mode format. position == current xyz, target == target xyz, 
-// offset == offset from current xyz, axis_XXX defines circle plane in tool space, axis_linear is
-// the direction of helical travel, radius == circle radius, isclockwise boolean. Used
-// for vector transformation direction.
-// position, target, and offset are pointers to vectors from gcode.c
-
 #ifdef __AVR_ATmega328P__
 // The arc is approximated by generating a huge number of tiny, linear segments. The length of each 
 // segment is configured in settings.mm_per_arc_segment.  
@@ -155,4 +149,4 @@ void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, ui
 void mc_go_home()
 {
   st_go_home();
-}
+}