Adds joystick support.

First two game controllers control the players in addition to the
keyboard. Analog axis to move, any button to grab.

Potential problems:
- No dead zone. Controllers with jitter might show phantom movement.
- No handling for more than 2 controllers; may crash w/bad array index.
Merge pull request #1 from mildmojo/add_joystick_support

Add joystick support