* Added getDomainMaxScale() and getDomainMinScale() to JS api.
* Updated scaleAvatar controller module to use this to prevent scaling past the limits.
* Made sure that getDomainMaxScale() getDomainMinScale() and getUnscaledEyeHeight are thread safe,
so that they can be invoked on the script thread.
* Added signals to Model class that can be used to let observers know when the Rig has finished initializing it's skeleton.
and also when the skeleton is no longer valid. These hooks are used to cache the unscaled eye height of the avatar.
