Document on_failure(), on_killed(), on_success() Methods

[bownux]

Just wanted to pointed out until I get some time to fork the project and contribute changes myself.

Suggestion:
After "Also, all the status setting methods take any number of hash arguments" it should also describe the method handling for each.

Reason:
I imagined I could set the status to failed by calling status setting methods from within a job. But I noticed

completed unless status && status.completed?

Would just set the status back to completed. Then I realized it should raise() an error, and that I could create on_failure() methods to handle it and still have access to the status object.

Thank You.