Add :document_counter to index.atom.builder

In my application, I've done some pretty extensive customization of the catalog#index view, and as a result, I need to access the 'document_counter' variable in some of the templates where it wasn't originally included.

As a result, the Atom links are now causing errors in my app, because in line 69 of app/views/catalog/index.atom.builder:
```ruby
xml.text! render_document_partial(doc, :index)
```
'render_document_partial' is called without the :document_counter parameter, and the templates in my app that are rendered by 'render_document_partial' expect that the :document_counter parameter will be available. And when it isn't, the app throws an error.

I believe that this problem may plague other Blackight apps that have customized the catalog#index view, though I can't be 100% sure since they're running in production. Some examples:

* Columbia University Human Rights Web Archive: http://hrwa.cul.columbia.edu/search.atom?q=human&amp;search_type=find_site&amp;search_type_mobile_button=find_site&amp;utf8=%E2%9C%93
* Tufts: http://dl.tufts.edu/catalog.atom?q=murrow&search_field=all_fields&utf8=%E2%9C%93

(Obviously there could be other issues causing errors with these sites/links.)

This pull request fixes the problem and doesn't affect the pre-existing functionality.