Allow for subscribing or callbacks on future events

[philipashlock]

Phil: In an early draft system architecture diagram, I'd included the idea of using a PuSH feed to keep updates propagated, so perhaps that kind of approach would be feasible in the future: http://wiki.open311.org/Open311.org_Draft_Spec#System_Architecture

Kam: In an early proposal draft I had included call back URLs that would receive an Open311 provider compliant call. My vision was that the call back would be the same as the call to the originating provider that caused the event. I think some municipalities were concerned with either security and/or complexity in the design and it got dropped.

http://lists.open311.org/groups/discuss/messages/topic/2C7IeNlIyfyEUG46UR5D05

http://lists.open311.org/groups/discuss/messages/topic/5LN3FHSq0qFDk8cHOHt9ND

Also related to tickets #28 and #51