THREE.MinecraftCharBodyAnimations Update - LegZ Rotations and Default Positions

I'm not sure if this is in the scope of what you want to achieve - so feel free to reject this patch.  I found it useful, though - as I could create a bit more realistic representation of horizontal ( strafe ) movement and a great falling animation by adding legZ rotation.

I also went ahead and presumed a default position of 0 for anything not added to an animation step ( more or less the same behavior as before ) but this allows you to not define rotations that are not changing.