Expose Server::many for creating a Server that will listen on many (ip, port)
pairs.
Handler still receives a simple Iterator of (Request, Response) pairs.
This is a breaking change since it changes the representation of Listener,
but Handler and Server::http are unchanged in their API.
Fixes#7
This introduces a new Trait, NetworkStream, which abstracts over
the functionality provided by TcpStream so that it can be easily
mocked and extended in testing and hyper can be used for
other connection sources.
