Momentum Physics

Added momentum physics and made enemies stay within the bounds of the
screen. We still have some work to do on the enemies. I believe true "homing" enemies should estimate the amount of time it's going to take to reach their target, and then aim for where the player will be in that amount of time, instead of aiming for where the player currently is.

In the current system enemies tend to "orbit" the player sometimes. Still a hard game though, even without true homing.
@zacharyw Let's split this method up if we can.