Allow resetting of tag_lists and cached_tag_lists

We have a tag admin area and when a tag was renamed we didn't have an easy way to update the associated cached tag lists of objects with that tag. This patch doesn't add the functionality directly, but it does give a helper method so you can loop through object.tagged_with(some_tag) and then refresh the object's cached tag list. I'm open to add more of this into the gem too, but wanted to start small and see if there is interest first.

Also, this is my first patch for this project so any feedback you have on code organization, format, or anything else, just let me know. Thanks!