Be consistent in setting of the fan PWM.

Always set the FAN PWM value in check_axes_activity() and always use
the value of the active move if a move is active.
Add "kick fan" feature - briefly run fan at full speed on start.

Add a feature to run the cooling fan at full speed for a small period
(default 100ms) when first starting the fan.  Some fans wont reliably
start spinning at low power, and many fans have issue with the PWM at
low power.  However, once the fan starts spinning it can reliably be
set to a wide range of PWM values.
Merge pull request #334 from KevinOConnor/FanKick

Add feature to briefly run the cooling fan at full speed when it first starts.