Added TankSprite; getInstance() static method to Graphics

- The Graphics.getInstance() static method is only accessible to the
graphics system, and it must not be called before the Graphics class is
explicitly instantiated.
- Enables all graphics classes to access the Graphics system, even if
they don't have a reference to it
- TankSprite now sets the tank camera controller. However, it will need
a way to determine if the tank is the local player before it works
properly.
- Added additional tests
Added main game class and loop

- Main game class: GameApplication
Fixed FindBugs issue in Graphics