Add support for parameterized resque jobs.

Parameterized jobs are defined in ```config/resque_schedule.yml``` by giving the job a ```parameters``` key. Each parameter may have a default value defined. Parameters may also be set along with args.

Parameters may also have a description. If a parameter has a description defined, the description appears on mouseover of a (?) that appears next to the parameter name.

```yml
backfill_logs:
  class: Jobs::Backfill
  args:
    api_key: '1234567890'
  parameters:
    app_id:
    severity:
      description: 'Minimum log severity to fetch'
      default: 'warn'
```

If a resque job has parameters defined, when you click on the "Queue Now" button in the Schedule tab, it takes you to a page to set the parameters. A resque job is then queued with the set parameters.

[Screenshot](https://skitch.com/rlipman/8i83x/resque)

[Screenshot with description (shows up on mouseover on the ?)](https://skitch.com/rlipman/8iq1b/resque)

Inspired by the [Jenkins parameterized builds plugin](https://wiki.jenkins-ci.org/display/JENKINS/Parameterized+Build)