Additional Work 8/6/2017

Walls now deregister their effects when they are destroyed.

Creep powerups at sources have been modified in a couple of ways:

1. The creeps new remember that they're under the influence of the source, and don't repeat the power up each step through the source.
2. The powerup is now handled in the source object, allowing different effects to occur based on the source type.  Right now, all that happens is the speed and hits of the creep change differently for different color sources.
3. Each creep has a chance to damage the source that powers it up as it passes through.

The creeps now explode when they hit the target source, exploding with their remaining hits in their fury.  I had to stop the source from taking hits because the game can't handle it right now, but the spoke animation works correctly: when enough hits are taken, the spokes turn from the source color to gray.