Xbox controller script

This is targeted at controller use with the DK2 headset, so:
  * No acceleration, speed is constant.
  * Walking is relatively slow and is meant for smaller environment like the bar scene

Right now controller support is hard-coded to the Xbox 360 controller on a Mac (on Windows it doesn't fully work).  @birarda and I have talked about moving to SDL2 for hot plug support, which also includes input mapping which should make the task of generalizing a lot easier.