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
Update client benchmarks for new Listening representation.
Split Server::listen into two methods to hack around ICE related to default type params

Trying to default the type parameters leads to an ICE and strange type errors.
I think this is just due to the experimental state of default type params and
this change can be rolled back when they are fixed.
Merge pull request #25 from reem/server-bind-multiple-addresses

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