Finished Powerups

Hey everyone, I finished powerups. However, since they have yet to be
tested in multiplayer, we might want to keep it out of our Alpha
Release. So I will continue to only post it in the Powerups branch. At
the moment, we have Shield (lasts 30 seconds), Rapid Fire (lasts 30
seconds), and Infinite Energy (lasts 15 seconds). These values can
change at any time. If you have any more powerup ideas, let me know.