Include declustered entities in cluster callback

Requested on the Forum here: https://groups.google.com/forum/#!topic/cesium-dev/hUrOxJQjjzg

They would like a way to access the entities that have been unclustered in the cluster callback function when the event is fired.

Start by reading the issue and the linked Cesium forum request to understand the desired cluster callback behavior. Then locate the clustering callback entry point and its tests in the repository. Done means the callback exposes the entities that were unclustered, with coverage for the event behavior.

Written by the indexing model from the issue text.