Laser coolant flow sensor support

This adds code to the Laser module to check a coolant flow sensor that emits a pulse stream while water is flowing - see http://www.thingiverse.com/thing:15415 for a DIY sensor of this type.  This has been briefly tested on actual hardware.

In the process of developing this, I added some new Pin methods for working with the edge detect capabilities on ports 0 & 2, and a "gpio" shell command for debugging.