Added Docker client autoConfiguration

Hi,

I created autoConfiguration for the docker-java [1] Docker client. In the most common scenario, adding docker-java to the classpath will provide `DockerClient` instance connected to the `$DOCKER_HOST` environment variable:

    @Autowired
    DockerClient docker;
    ...
    docker.pullImageCmd("ubuntu").exec();

Cheers.

[1] https://github.com/docker-java/docker-java