Change the MuxedTransport trait

[tomaka]

Based over #80

Right now the MuxedTransport trait provides three methods: dial, listen_on and dial_and_listen. The difference between dial and dial_and_listen is that the latter allows you to handle the substreams that are opened by the node you dialed.
Whenever you use a muxed transport, you are strongly encouraged to use dial_and_listen instead of just dial.

This PR simplifies this design by removing dial_and_listen entirely, and replacing it with a next_incoming method that will consume the transport and return the next opened substream. Since the transport is generally clonable, you can easily turn that into a stream of incoming connections.

[folsen]

I have nothing to complain about, the API seems much cleaner and more intuitive.