Spindle speed overrides behavior tweak. New experimental laser dynamic power mode.

- Spindle speed overrides now update immediately if they are changed
while in a HOLD state. Previously, they would update after exiting the
HOLD, which isn’t correct.

- New experimental dynamic laser power mode that adjusts laser power
based on current machine speed. Enabled by uncommenting
LASER_CONSTANT_POWER_PER_RATE in config.h

  - It assumes the programmed rate is the intended power/rate for the
motion.
  - Feed rate overrides (FRO) do not effect the power/rate. Meaning
that spindle PWM will automatically lower with lower FRO and increase
with higher FRO to keep it the same.
  - Spindle speed overrides (SSO) will directly increase and decrease
the power/rate. So 150% SSO will increase the PWM output by 150% for
the same speed.
  - The combination of FRO and SSO behaviors should allow for subtle
and highly flexible tuning of how the laser cutter is operating in
real-time and during the job.

- Re-factored planner block rapid rate handling for the dynamic laser
power feature. Should have had no effect on how Grbl operates.

doc/log/commit_log_v1.1.txt

@@ -1,3 +1,19 @@
+----------------
+Date: 2016-10-26
+Author: Sonny Jeon
+Subject: Add high-frequency spindle output option. Minor parking motion re-factoring.
+
+- Some laser controllers were reported to need a very high PWM
+frequency. Added a line to enable this in cpu_map.h, if needed.
+
+- Cleaned up some of the parking code. Mostly just editing the comments.
+
+- Moved the accessory state resetting after the initial parking
+retract. Should ensure the accessory state is properly handled upon an
+aborted parking restore. Not certain if this was a problem before
+though. Just to be sure.
+
+
 ----------------
 Date: 2016-10-25
 Author: chamnit