Merge pull request #2 from dmaze/master

TaskMaster.get_work(): get one work unit and stop...
Split out base "thing that talks to Redis" class from registry
Redis-based priority queue.

Supports expiration and item locking.  This is separate from registry/
task master.  You can add items to the queue with some priority, then
fetch out the highest-priority item (holding it for some time period),
reserve other items (attached to the original checked-out item),
and return things to the queue when done.