support for unix sockets

hey, 

This provides support for unix sockets as requested in https://github.com/docker-java/docker-java/issues/20

You can connect to local docker by using "unix://localhost/var/run/docker.sock" as URI. 

Please mind that I did not write this code. The https://github.com/spotify/docker-client/ project did, I copied over the relevant code snippets and classes from their source tree. Both projects use the apache 2 license, so I think this should not be a problem. Fell free to overrule me, if your legal knowledge is better than mine.