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.
@rohansingh Line 89 below has `http` hardcoded. So even if the `raw` param passed into `normalize()` is `tcp://192.168.59.103:2376` which is the HTTPS port, `http://192.168.59.103:2376` will be returned. This causes [this line](https://github.com/spotify/docker-client/blob/fec96c80fca10dcdf9ddc7440c530f54b3da4cf2/src/main/java/com/spotify/docker/client/DefaultDockerClient.java#L190) to throw later on.