Add support for docker commit method (commit container with changes as new image)

Added to resolve https://github.com/spotify/docker-client/issues/76
This method allows you to use the docker client to commit a new image based on an existing container.
See: https://docs.docker.com/reference/api/docker_remote_api_v1.15/#create-a-new-image-from-a-containers-changes
Docker API doesn't specify this as far as I could see, I might be wrong of course... You could always send an empty string (not null). 
Possibly I can add another method, wrapping this one (keeping it public) with the properties that aren't required, such as comment. What do you think?