JointState::_rotationInParentFrame is now private
add "visible" transforms to JointState

also stubbery for using them in Model and SkeletonModel
Add Model::inverseKinematics() for stable hand IK

(removed constraint enforcement which was causing problems,
will add that back later)
merge upstream/master into andrew/ragdoll

Conflicts:
	interface/src/avatar/SkeletonModel.cpp
Merge pull request #3109 from AndrewMeadows/ragdoll

stable arm IK without constraints