cleanup: JointState transitions toward a proper class

I'm going to be adding some intelligence to JointState to support simultaneous animation and collisions.  This is partial progress toward that goal: 

(1) JointState is no longer inside the Model namespace (although it is still in the Model.* files -- it will be moved out soon)
(2) it has a pointer to its corresponding FBXJoint
(3) it has a new method called updateWorldTransform() which knows how to do the math and helps clean up the Model code.