Skeletal Animation Blending

Reintegrated and improved upon the previous work.  I've added a small and simple example to test usage.

Initially, the Animation object did some manipulation of the blend weights for fade in and fade out but it became clear to me that it would be much cleaner and more flexible to have other pieces of logic dedicated to that (ie blend trees).  I figure over time people using it will converge on a good api that can then be integrated when the time is right.  For now, any code is free to manipulate the weights as they please.