Added a Game class
Created a Game class that runs the game.

Temporarily removed collision handling, because it needs to be
integrated with the Game class.
Moved collision handling into the Game class

Collision is now handled by each object, and the Game class
automatically checks for collisions between all objects with colliders.

The ground has been turned into a Box object. Again, this means that
collision with the ground is automatically handled.
Configurable camera movement.

Every GameObjects draw-method now takes a position parameter, which is a
Vector2 telling the objects position on the screen.

The Game class now takes a camera, which it will use for translating
world positions into screen positions. This camera can, of course,
be moved around like any other GameObject.
Seperated library code from game logic
Better camera movement, and a bit of cleaning out.
Integrated Thadeus Burgess' input handler.
Actual gameplay! Major changes:
- Added a single power up
- Cleaned up a bit in the collider module - it's much nicer with a
  BoxCollider which actually takes a rect.