Allow sort fields to be re-ordered and labels to be updated

Closes #675 

![sort](https://cloud.githubusercontent.com/assets/96776/5408417/f941aaf6-818e-11e4-8e27-fffb89c3d611.gif)

Instead of passing the solr sort string in, what about passing in the config object? I could foresee applications wanting to override this behavior and giving them the config might make that easier (e.g. maybe someone could stash a `config[:description]` to use instead)