Introduce a ShuffledRoundRobin reserver.

This ensures work is done more evenly across all queues. The
normal RoundRobin reserver always slightly favors the first
queue in the list since all workers will start at the same
spot everytime they get restarted.
Merge pull request #82 from seomoz/shuffled_round_robin

Introduce a ShuffledRoundRobin reserver.