Add the ability to sort results

I've added a basic sorting (based on the [jsonapi spec](http://jsonapi.org/format/#fetching-sorting)).

For now it only supports sort (not sort[DOCUMENT_TYPE])

I also haven't added any default (eg. on the primary key), because I think it should be opt-in by the user/developer if she want to have records sorted on it or not. What do you think?