unix socket support

hey,

This provides support for unix sockets as requested in #20. In contrast to my last pull request, I fixed some bugs. 

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. Feel free to overrule me, if your legal knowledge is better than mine.

The integration test suite does not work for me, however, it never did so it is unlikely that I broke it. Maybe someone can check that before merging? All other tests, including my own test for using unix sockets, do work as expected. 

The file PortsTest had some \r line endings, which git fixed automatically somewhere along the lines, so the file shows up as changed. However, the only difference is whitespace characters.