Cleaned up Package arrangement to work with the PowerUps branch.
Renamed package from PowerUps to Items.
Had Items that were already made implement the Item interface that our
branch has.
Fixed HashMap declaration in PowerUpList and added comments for that
group.

Removed old PowerUp interface under the assumption that these test items
made would be Items and not PowerUps. (Items get added to an inventory
whereas PowerUps are used automatically). This doesn't affect anything
because nothing used that interface.