Quaternion now has a new property onChangeCallback which, if set, will be invoked each time the quaternion is updated. This allows you to link change events to other objects.

Internally, the `Quaternion` class now has 4 new private properties: `_x`, `_y`, `_z` and `_w` and 4 new getters and setters for the public versions. It also now passes most methods via `set` to allow for the onChange callback to be invoked. This does not change the public-facing API.

Author: photonstorm