Added Listener object to the API

[philsbln]

This adds an explicit Listener object to the API (closes #301) to

• clarify behaviour of multi-streaming protocols (closes Clarify multi-streaming can cause that ConnectionReceived events can be fired from pre-connection used for initate() #269)
• clarify behaviour and enable re-use of Preconnections (closes Reusing Preconnections? #222)

[mwelzl]

This looks just right to me, thanks a lot for doing this!
In particular having read #269 again, I believe that @tfpauly should definitely have the last word on this one. So I'll go with whatever @tfpauly says.