Add ActiveRecord::Relation#destroy_all! method

The ```destroy_all!``` method is useful for when you want to destroy a collection of objects but want the behavior of ```destroy!```.  If the ```before_destroy``` callback returns ```false``` for an object being destroyed an ```ActiveRecord::RecordNotDestroyed``` error will be raised.

I couldn't think of a way to keep this DRY with respect to the ```destroy_all``` method without getting too fancy, suggestions welcome of course!