Implement Momentum

[ccallebs]

@zacharyw Any ideas? I'm not quite sure how to tackle this particular problem.

[zacharyw]

I think we need an acceleration vector that alters velocity, instead of setting velocity to a constant amount.

[zacharyw]

I've implemented the acceleration vectors. Now we need to implement some homing functionality for the enemies. They need to detect how much time it's going to take to get the player based on if the player is moving away from or towards them (or sitting still) and then aim for the spot where the player will be, instead of where they are.