Start of working to use more powerful Elliptics features

to improve performance.
There've been already implemented:
* get_size: lookup.size is used instead of stupid len(read)
* exists: trying to read the first byte to enforce quorum check
Catch exceptions.FileNotFoundError according to new docker_registry policy
get_size must throw exceptions.FileNotFoundError in case of

inexistent key.
Merge pull request #6 from noxiouz/elliptics_features

Start of working to use more powerful Elliptics features