Support for automatic re-sizing of EC2 pools

The final settings will probably need tuning as we watch how it works, but this seems like a good start.
Instead of creating dummy classes we can manually enqueue a class name like this `Resque.enqueue_to(worker_thresholds[:autosize_queue], 'ShutdownInstanceJob')`. Is that confusing because you can' tell where ShutdownInstanceJob is coming from?