Use a redis set to store queue names, instead of relying on the redis "keys" method,
which does not perform well on large databases.
Only save queue to set on enqueue.
Validate queue key still exists when all() is called
Merge remote-tracking branch 'upstream/master'

Conflicts:
	tests/test_queue.py
Remove misleading test.
Record new behaviour of Queue.all() explicitly in a test.
PEP8ify.