Since the test suite `flushdb()`'s after running each test, we should
make sure the database is empty before we even start running tests.
This patch will make sure to never destroy any local production data
inside the running Redis instance.
This fixes#25.
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.