Add ChangeDetectorAgent, test

New agent will watch the property of an inbound event stream. When that
property changes, it will emit the inbound event.
Updates based on feedback

* Updates the default option for `property` fro 'output' to '{{output}}'
* Use Delorean for tests
Merge pull request #437 from PetroFeed/add_change_detector_agent

Add ChangeDetectorAgent, test