Add `pool_size` for adding multiple delayed_job workers

This pull request allows you to specify a `pool_size` for the worker count. I've also added a `workers` key for manually configuring options for each worker.

For example, setting a `pool_size` would setup workers: "delayed_job.1, delayed_job.2, etc." under monit

    "delayed_job": {
        "pool_size": 6
    }

You can also manually configure workers by omitting pool_size and using the `workers` key

    "delayed_job": {
        "workers": [
            {
                "identifier": "regular",
                "timeout": 60
            },
            {
                "identifier": "priority",
                "options": "--min-priority 10",
                "timeout": 30
            }
        ]
    }