Remove a Delayed Job

It would be great to have an equivalent to the "remove_delayed" function of the original Resque scheduler.

``` ruby
# after you've enqueued a job like:
Resque.enqueue_at(5.days.from_now, SendFollowUpEmail, :user_id => current_user.id)
# remove the job with exactly the same parameters:
Resque.remove_delayed(SendFollowUpEmail, :user_id => current_user.id)
```
