allow for PropertyBindings to bind/unbind.  PropertyBindings are reference counted by Mixer for efficiency.

only update scene graph values if actually different, only call triggerDirty if values changed.  Use bone names.

fix bugs in KeyframeTrack.optimize/validate.  Expand capabilities of PropertyBinding bind to work with bones.  fix bugs with JSONLoader animation converter.

memeoization of lerp.  add optimize, trim, validate, sort to KeyframeTrack.  Create separate AnimationClipCreator class to get demo code out of ThreeJS src/ directory.