Jobs are now stored in separate keys, and only job IDs are put on Redis
queues. Much of the code has been hit by this change, but it is for the
good.
No really.
There is no job tuple anymore, but instead Jobs are picklable by
themselves natively. Furthermore, I've added a way to annotate Jobs
with created_at and enqueued_at timestamps, to drive any future Job
performance stats. (And to enable requeueing, while keeping hold of the
queue that the Job originated from.)
This fixes#17.