Support unix domain socket

em-proxy supports unix domain socket. (https://github.com/igrigorik/em-proxy/blob/master/spec/proxy_spec.rb#L210)

Then, I have tried to support unix domain socket on kage.
With this feature, we can use kage with servers supported unix domain socket like nginx.

If kage supports ruby 1.9.3, I should use options hash instead of keyword arguments.
Do you think about?