First implementation of minimal Broker API

Yes, names are tricky. Hence I won't interfere to much. But as stated elsewhere, I doubt the "events" part. hbpvis::messaging was a crude guess. Yet, that's what you get when using namespaces the hard way. I could live with a three letter acronym like msg as well. I wouldn't want to tie it too closely to zmq now because that might change in the future.