Added support for setting delayed_job queue

This PR adds support for setting the delayed_job queue name in the options passed to `process_in_background`. For example: 

```Ruby
process_in_background :avatar, :queue => :avatar_queue
```

The primary use case is to allow the configuration of delayed_job workers dedicated to processing an individual queue, i.e. you may wish to have one worker processing profile images while another is processing product images.