title: Group records on activeadmin resource index page
body: <p>I have 2 resources - Shelf and Book, where each Shelf has many Books.</p>
  
  <p>Is there any way to groups books (on Book index page) by Shelf? Specifically I would like to have Books sorted by Shelf and whenever Shelf changes, I would like to indicate that on the books list (inserting a h3 title with new shelf name between books on one shelf and books on the other)</p>
  
  <p>I am using rails 3.2 and activeadmin 0.6.0.</p>