Bug fixes, backgrounds, HUD, states, graphics.

- Fixed a bug where the end level state was showing the enemies killed
for level 2 instead of level 3.
- End level state now has a background.
- Fixed the mute music button doing the opposite of what it was supposed
to do.
- Buttons now have pretty graphics, including a mouseOver state.
- Added a non-parallaxing layer to the background of every level state.
- Added a HUD.
- Fixed levels accumulating the completion time of the previously
completed levels rather than starting from 0.
- Added a whole bunch of assets to build levels with.
- Added a credits state.
- Added a help/instructions state.
- Fixed player being able to attack while holding Hitler.
- Levels now have 2 separate mute buttons. One for sounds and one for
music.