removal of unused getNeckRotation()
remove unused Head::_gravity data memeber
merge upstream/master into andrew/inertia
move hand/arm stuff from Model to SkeletonModel
move face/head code from Model into SkeletonModel
fix bug -- negate logic around invalid joint index
simplified hand index constants
Merge pull request #2935 from AndrewMeadows/inertia

cleanup: move avatar specific stuff from Model to SkeletonModel