another quick update

Decided to break collision detection out into it's own class. To add
collision detection to your own objects, implement the Collidable
interface with your object and pass it to Collision.Detect().
Collision.Detect() will do all the heavy lifting and return true if a
collision is detected, and false if a collision isn't detected. See
Pigeon class for how to implement Collidable, and
PigeonManager.checkCollisions() for how to call Collision.Detect().