RagDoll Part 1: basic data structure

This adds a RagDoll class to the project.  The RagDoll has a list of points and list of DistanceConstraints between them.

SkeletonModel has a _ragDoll member that is initialized with the _jointStates (in their bind pose).  Each frame the _ragDoll  is slaved toward the JointStates and then the constraints are enforced.

No collisions yet.

Also: splitting JointState class out of Model.* and into its own files JointState.*.