Migration to Box2d for Physics

[Twinklebear]

Need to get more familiar with Box2d and do more testing with it to see how it will integrate

Physics class will probably end up as an in-between for an physical entity and its box2d components.

In addition need to change Physics::SetMove function to take a vector direction to move in

[Twinklebear]

I've been playing around with a test bench of SDL + Box2d to become more familiar with the library. I may add it into an experimentation/testing branch.

[Twinklebear]

Noticed while going through physics.h, that the ground friction force is a hard coded constant. It should vary depending on the type of tile the entity is on.

Rather a minor issue for now, but still something to change. Perhaps wait til Box2d is in to look into this?