Add negative bang instance methods

I added a reverse *bang* method. So for example, if we have an `archived_at` column, we would have both `archive!` and `unarchive!` (alias from `not_archive!`) methods to toggle a record archive state.

![](http://25.media.tumblr.com/bf5384db70aeee86684236ca51f0a7cc/tumblr_mibdb7Z2q81r0ujc2o1_500.gif)