Controller mappings for PS3 (official and generic). 

Also added XNA-style ToString() overrides to a few of the GamePad subordinates to aid in debugging.

Only tested on OS X, using official PS3 controller, and a GameStop knock-off PS3 controller.

Issue: https://github.com/mono/MonoGame/issues/2154

Rationale: Using PS3 controllers in OS X is a breeze compared to the 360 -- especially in Mavericks, where the tattiebogle drivers don't seem to work any more. Nothing to install, just plug the PS3 controller into the USB port. (Even the wireless works with a mini USB cord.)