Beginnings of Entity Hierarchy

Base Entity class created, from which all positionable "Game Objects"
will inherit from.

DXMath class created, holds Vector and Quaternion structs/classes which
have implicit casting operators to various DirectXMath structs. Thus,
there is no longer a need to juggle between XMVECTOR and XMFLOAT
structs.

Created Crate class. This class is going to change a lot, as variables
will move in and out of it as I continue the Entity hierarchy. The crate
spins on the screen using quaternion math.