PWM Spindle Control and Invert Spindle & Coolant Pins

Updated version of Pull request #303 to use Timer2
These changes are mostly untested

********* Details from #303
I have added PWM to the Spindle, you can set the spindle as either PWM or Stepper.

On the UNO the Y Limit switch has to be disabled to use the PWM on that pin.
On the Mega Timer 4 has been used so it does not affect the limit switch pins.

Status:
PWM - Tested and working
Stepper as Spindle - Tested on Scope only ( but looks good )
Invert Spindle Enable Pin - Tested on Scope
Invert Coolant Pins - Tested on Scope

A Warning has been added to pinmap.h about the invert pins will be active for a moment while the controller is resetting.

These updates refer to issues:
#284 PWM to Spindle
#258 Stepper as Spindle
#218 Invert Spindle and Cooling Pins