Implement basic physics + entity system

[fesoliveira014]

A basic physics system should be simple in the context of a block world. Basically it will consist of a gravity mechanism and collision checking. The logic for box collision checking is already in place, and so is a primitive entity system. The entity system treats each of it's instance as an rigid body, and can perform transforms on that object, such as rotation, translation and scale. The physics system would use these methods to implement basic newtonian physics on the blocks. For better physics, it would be necessary to use Oriented Bounding Boxes instead of AABBs as we are now, but implementing OBB should be as simple as making transforms over a AABB (see Giawa's Bounding Box implementation).

[fesoliveira014]

Entity is done, physics on hold.