Homing and feed hold bug fixes.

WARNING: Bugs may still exist. This branch is a work in progress and
will be pushed to the edge branch when at beta stability. Use at your
own risk.

- Homing freezing issue fixed. Had to do with the cycle stop flag being
set incorrectly after the homing cycles and before the pull-off
maneuver. Now resets the stepper motors before this can happen.

- Fixed an issue with a rare feed hold failure. Had to do with feed
hold ending exactly at the end of a block. The runtime protocol now
sets the QUEUED and IDLE states appropriately when this occurs. Still
need to clean this code up however, as it’s patched rather than written
well.

- Updated version build via $I command.

- Forgot to comment on a new feature for the last commit. Since steps
are integers and millimeters traveled are floats, the old step segment
generator ignored the step fraction differences in generating the
segment velocities. Didn’t see like it would be much of a big deal, but
there were instances that this would be a problem, especially for very
slow feed rates. The stepper algorithm now micro-adjusts the segment
velocities based on the step fractions not executed from the previous
segment. This ensures that Grbl generates the velocity profiles EXACTLY
and noticeably improves overall acceleration performance.

spindle_control.c

@@ -61,11 +61,11 @@ void spindle_run(uint8_t direction, float rpm)
 
   // Halt or set spindle direction and rpm. 
   if (direction == SPINDLE_DISABLE) {
-  
+
     spindle_stop();
-  
+
   } else {
- 
+
     if (direction == SPINDLE_ENABLE_CW) {
       SPINDLE_DIRECTION_PORT &= ~(1<<SPINDLE_DIRECTION_BIT);
     } else {
@@ -80,7 +80,7 @@ void spindle_run(uint8_t direction, float rpm)
       if ( rpm > SPINDLE_RPM_RANGE ) { rpm = SPINDLE_RPM_RANGE; } // Prevent uint8 overflow
       uint8_t current_pwm = floor( rpm*(255.0/SPINDLE_RPM_RANGE) + 0.5);
       OCR_REGISTER = current_pwm;
-      
+    
       #ifndef CPU_MAP_ATMEGA328P // On the Uno, spindle enable and PWM are shared.
         SPINDLE_ENABLE_PORT |= (1<<SPINDLE_ENABLE_BIT);
       #endif