Handle tcp:// URLs

This means docker-py will do the Right Thing if the base url is set
using the new `DOCKER_HOST` environment variable, for example.
(http://docs.docker.io/en/latest/commandline/cli/)
Accept base_url of None

This makes it very simple to get identical behaviour to the CLI:

    client = Client(os.environ.get('DOCKER_HOST'))
Merge pull request #140 from aanand/sensible-url-handling

Sensible url handling