Update docker.Client to default to DOCKER_HOST so that it "just works" more often

Also, update docker-py requirement to 0.4.0.

See also https://github.com/dotcloud/docker-py/pull/204 (especially the discussion there).
Merge pull request #124 from tianon/DOCKER_HOST

Update docker.Client to default to DOCKER_HOST so that it "just works" more often