Level one

this is an implementation of the "level one" stage.

In this commit:
Changed the physics engine.
Changed game mechanic. Larger balls accelerate towards you. You must avoid them. Smaller balls do too, but at a slower rate.
Implemented a five second invincibility state since player was getting spawn camped at game start.
Added keyboard controls.

Everything else is the same. The last five commits are just cleaning up the code for review.