Jesse, thanks for capturing my e-mail thread in this issue.  Can you 
comment on my last three paragraphs?  Essentially, I think we should 
lock down the API and assert that Listener.address will always be 
a 'connectable' end-point.  (i.e. not a wildcard host, 0.0.0.0, that 
can't be bound to by a socket, for example)

This would mean raising an exception in Listener.__init__ if this 
invariant is violated.