Change Server to contain a Vec of (ip, port) pairs to allow repeat

listening

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
So, this swallows IoErrors, right? Something simple like "addr is in use" could now be difficult to determine.