Make Entity available in callback function for CallbackProperty

Suggested in the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/tA0lH-ZuF8Y

It would be useful if `this` or some other variable represented the calling entity to avoid duplicate functions for each entity.

Start with the CallbackProperty implementation and the linked forum discussion to determine how the calling Entity should be exposed to callbacks without requiring duplicate functions. Done means a callback can identify its calling Entity, with tests covering the resulting behavior.

Written by the indexing model from the issue text.