Add the :touch to update ancestors on save

This is analogous to the same option on Rails' `belongs_to` association. I needed it because I'm using nested [key-based cache expiration](http://37signals.com/svn/posts/3113-how-key-based-cache-expiration-works) to render some big trees, and since modifying a child node doesn't touch its parent, my child nodes' fragments were never being re-rendered.

I'm currently doing this with callbacks in my models, but this seems like it could be useful elsewhere (especially with Rails 4's preference for nested fragment caching), and the default behavior doesn't/shouldn't change at all.