First implementation of minimal Broker API

1. fine with me for now, but I might turn adamant in a future revision ;-)

2. I guess this is a point that's hard to clarify in (reasonably) short text but easy in a f2f discussion. Including the handler/routing in here makes it "automagic". This is convenient at first sight but limits reuse down the road. Multiple handling/routing strategies might all use the same receiver implementation. To clarify: I'm not against predefined routers/handlers, I just believe that it's too early to hard code them at this point. That's all.

3. depends on whether you count the c'tor as a method ;-) Again: simplicity of the interface rules...