Powerup added

Right now, I have only created a shield powerup. And at the moment,
there is no graphical change for the shield. I attempted to get the
purple shield model working, but it never showed up. In any case, I am
heading to bed. I will make other powerups tomorrow. Also, if we decide
powerups are not going to work in the long run, we can scrap this branch
or save it for later.
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.
Merge pull request #32 from hollsteinm/Powerups

Powerups