Natural version sorting

Requested on the feedback forum: http://goo.gl/vP9TyR and http://goo.gl/uYUXrw

Makes use of the [version_sorter](https://github.com/defunkt/version_sorter) gem, which uses a C extension, so it should be nice and speedy.

Additionally, the grouped_options_refs helper had zero tests, so I added some.