Paginate custom collections

Hey guys,

I tried to setup pagination for custom collections and found that this is currently impossible.
I found homebrew solution here http://forum.middlemanapp.com/t/pagination-with-custom-article-collections/1498, but code change is pretty easy to allow pagination for custom collections in elegant way.

Cucumber `pagination.feature` is updated.

To test this I updated `paginate-app` fixture, not sure this is a good solution, I ready to create new fixture app for testing paginated collections, just let me know.

Thanks!