Support HTTP(S) sockets and docker-api's defaults

* Accept `http` and `https` socket types in addition to `tcp`. I was experimenting with running an HTTPS proxy in front of the Docker API. Everything seems to work once this is applied.
* Use `Docker.url` instead of hardcoding the same unix socket it defaults to. This allows support for the `$DOCKER_URL` environment variable that docker-api checks for.