Replaced Collision::Util with Collision::2D. Bullet changes.

Since we want to render moves before doing the collision detection,
detection is performed on the last move.

Some elements of the move method were placed in Thing to accomodate
this.

Bullet existence is no longer controlled by queue length, they now
cease to exist when they go "way_out_there" or hit something.
Added audio playing on shoot / enemy death events.

Also, some wav files.