displays has_many association counts

This displays counts for a resource's relationships on the show and edit pages, if the counter is configured to true. I found this to be useful in my project and thought others might want this functionality, as well.

Here's a screenshot to see how it is displayed
![screen shot 2013-06-09 at 7 57 03 pm](https://f.cloud.github.com/assets/1294184/629703/bdd3af6e-d168-11e2-8b69-e0e1469dbd5e.png)