Worker can process many queues

The new feature in this release is the ability for a worker to process many queues. The worker will accept a list of queues to work on and will process the queues in a left to right order. For example:

$ QUEUES="first,second" bundle exec rake qc:work

In this scenario, on each iteration of the worker's loop, it will look for jobs in the first queue prior to looking at the second queue. This means that the first queue must be empty before the worker will look at the second queue.

See 2fefa5f for more details. And more generally read over #190 for an overview of the refactorings.