Add various classes

* Application: Manages game systems and logic.
* Object: Base class for everything (not all classes currently inherit this yet)
* Entity: Base class for any object that exists in the game world
* PhysicsWorld: Manages physics game system
* Body: Entity that exists within the physics world, has velocity and acceleration.
* Vector2D: Physical quantity with an x and y component.

Mostly a skeleton commit. The Body class may change, and perhaps be merged into the Entity class as shown in the engine diagram. However, not all entities need physics, such as lights and sounds, but at the same time there should be a unified class with which the PhysicsWorld class interacts.