Spindle speed control v0.8

Requesting that this branch be added to grbl/grbl to provide grbl users with an immediate solution for Spindle Speed control.  This branch does the following:

- Enables a PWM pulse from S0 through S255 commands on Arduino Pin 11
- Modifies the stepper to use Timer 0 instead of Timer 2 overflow
- Eliminates Pin 11 for use as input for a Z-axis limit switch, though I believe it should not be too difficult to move that to an analog input pin like A4.

I'm not suggesting that this code be merged to master, but I don't have the option of selecting a new branch.  I've updated the Readme.md to with details on the changes, and more information is also available here: http://www.andrewlsandoval.com/GrblSpindleSpeed/

I've modified as little code as possible from Master -- just enough to make this one feature work so that I can control laser intensity via spindle speed commands.

Thanks,
Andrew Sandoval