First implementation of minimal Broker API

O.k. so that's another thing that I would handle differently - but that's really just my personal opinion: I would use one subscriber for each incoming connection just because I like to know from whom I get stuff. Technically, this comes down to where you do the de-multiplexing: internally in the broker or on the API level i.e. in an extra Demux class. Beware though: if you use a single ZMQ socket for multiple subscriptions on the implementation level, you have (to my knowledge) no means whatsoever to inquire which of the publishers that you are listening to just sent you the last message. But the current imp handles this "correctly", already.