Added some additional powerups

They do not have unique icons. for now they just use the alien's
explosion sprite.
Slow: Reduces speed of all enemies on the screen.
Bomb: Damages all enemies on the screen.
Scoreup: Grants a score boost.
Their spawn rates are defined in globalvariables.

Current issue: Sometimes when shooting a scoreboost, you'll see quite a
few of the same powerup spawn at once.