Use add/delete pattern instead of gets/cas.

Simpler & shorter code, fewer roundtrips, does not produce garbage on
python-memcached level (cas_ids).
Suggested by James Haggerty <jamesh@freelancer.com> .
Allow disabling release safety check.
Implement blocking acquisition quadratic backoff.

Suggested by James Haggerty <jamesh@freelancer.com> .
Add timeout support for lock acquisition.

Suggested by James Haggerty <jamesh@freelancer.com> .
Distinguish between unresponsive memcached and failed command.

python-memcached is making this hard for "add".
Implement stand-in optional behaviour.

Suggested by James Haggerty <jamesh@freelancer.com> .
Make lock persistence optional.

Automate lock release on owner instance destruction.
Rework docstrings.