Huge commit:

Refactored the architecture of com.HiS.gameobject and com.HiS.physics packages:
- Added interface PhysObject
- Moved getPhysics() from GameObject to PhysObject
- Added abstract class PhysGameObject
- Modified GameRenderer.render() and PhysEngine.render() to take in List<? extends PhysObject>
- Refactored Horse to extend PhysGameObject