Graphics -> Production

Many updates:
- Added concrete GameWorld class. We can change the implementation in the future, but we need a concrete implementation of the World interface if we want to create integration tests tomorrow.
- Added CameraController, which controls the camera's position based on the tank's position (fully implemented)
- Updated TankSprite to work with camera controllers (In the header of my check-in, I put "added TankSprite", which is a mistake. I must have zoned out)
- The instantiated Graphics object can now be accessed by any graphics system object, even if they aren't currently holding a reference to it, via the new getInstance() method (related to controllers)
- Added additional Sprite.create overloads. They currently return MockSprites. I'd rather have them throw an UnsupportedMethodException when called, but then my unit tests would fail, which isn't allowed.
- Added GameApplication, which contains the main game loop and hooks to the libgdx graphics library
- Added many tests (thanks again for the EclEmma tip)