Provide on request complete callback facility

This patch provides the ability to receive a callback on the completion of a
request.  The callback takes place immediately before the request's resources
are released.
evhttp_request_set_on_complete_cb to be more specific about what the function actually does and usage
Update unit test to make sure that the callback happens after the output data is written
Merge remote-tracking branch 'asweeny86/on-complete-cb'