Introduce Elliptics backend for registry.

We use a Docker in our open-source PaaS Cocaine (https://github.com/cocaine) in Yandex. Also we use Ellptics as storage in our cloud. So I added support Elliptics as backend for docker-registry.

More info about Elliptics: http://doc.reverbrain.com/elliptics:elliptics https://github.com/reverbrain/elliptics