First implementation of minimal Broker API

If this class can do both, publishing and subscribing, I wonder why it is handled asymmetrically, i.e. why is the publisher URI a c'tor argument whereas the subscription has to be done via a dedicated interface. Clearly, there will be both: publishers which don't subscribe and subscribers which don't publish. Hence, the situation is pretty much symmetric and so the interface should be as well. (...which brings me back to the argument of clearly separated responsibilities from above).