Introduce a cool-off feature for lock deletion.

It is now possible to supply an options argument via `lk:del/2`. This commit
introduces the 'cooldown' option, allowing the client to release ownership of a
lock but delay freeing of the lock for some time, specified in milliseconds.

Resolves #3.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>