Add support for HTTPS & Unix sockets

* Respect the DOCKER_HOST and DOCKER_CERT_PATH environment variables and
  connect to Docker instances over HTTPS or via Unix sockets.
* Remove all DOCKER_HOST setting logic.
* All the logic has been moved to github.com/spotify/docker-client.
Merge pull request #31 from spotify/dxia/use-https

Add support for HTTPS & Unix sockets