Add callback for skipped jobs

It would be nice to have a callback so that when a job is skipped because of a lock, you can reschedule it or do whatever processing is needed.

I forked your gem and added this feature - https://github.com/GAV1N/resque-lock-with-callback

Let me know if you'd like to incorporate it into resque-lock, in which case I can put together a pull request.
