Introduce Elliptics backend for registry.

More info about Elliptics: http://doc.reverbrain.com/elliptics:elliptics
Fix pep8 warnings.

Rename ell.py -> ellipticsbackend.py.
Merge pull request #101 from noxiouz/elliptics

Introduce Elliptics backend for registry.
Add action for listing repositories
List tags
Fix return
Make repository listing endpoint actually list repositories

After reading up on the docker lingo, I figured out what actually needs
to be listed.