Bugfix: change ClientHandler interface to Router.

Because the ClientHandler interface was a reply socket,
it could only handle a single client at a time.

By changing to a router socket, multi-client functionality
is restored.

See the [ZeroMQ guide](http://zguide.zeromq.org/page:all#Recap-of-Request-Reply-Sockets)
for more information.
Merge pull request #28 from reenl/master

Bugfix: restore ClientHandler multi-client functionality.