Enable the relay and aggregate daemons to forward metrics using a message bus using a stomp client 

The implementation is done by using the stompest.async package for the stomp specific parts (that package is build on top of the twisted framework, so that seems to be inline with the way the carbon daemons are build).

I tried to follow the coding style already used as much as possible and made sure the change is backwards compatible.

Hope that this patch/addition can be merged, but if not then please let me know what needs to be done in order to get it committed.