Batch of new features and changes.

Python:
    added remove indexes interface to session;
    now elliptics.Id aggregates dnet_id instead of inheritance from key. It makes using elliptics.Id easier, more natural and clearer;
    new method elliptics.Id.from_hex(string) that created and returns elliptics.Id by converting hex string to dnet_raw_id;
    added push/reply methods to session that close elliptics-cocaine interface;
    added capped_collection interface to session;
    added elliptics.Handler that can be used with logging.Logger.

Iteration:
    added total_keys and iterated_keys to each dnet_iterator_response, provided these counters via all bindings;
    added sending keepalive empty dnet_iterator_response with status=1 each time when iterator has been skipped 10K keys in a row. It prevents iteration to be timeouted on the client-side.

Pytests:
    separated test_session into several test_session_* tests that covers subfeatures.
    added bone for testing indexes, iterator and monitor&statistics.
    added new write-read test cases.