WE HAVE LEVEL LOADING!

- Implemented level loading through XML files
- New bullet types
- New bullet patterns
- Moved the keyboard listeners into the player.
- Fixed lag from large amount of bullets (bullets no longer perform
outward collision testing)
- Added test level files
- Added code for loading screens (if we find that the level loading
takes too long, or when I implement network level streaming, e.g.
streaming level xmls from the interwebs)
- Entities have to be registered before they can be spawned.