Add setting to change sort order of articles

Hi @mboinet,

I finally found some time to solve issue #21 :smile: The `comparator` and `checkboxradio` thing took me some time to figure out (first time using Backbone and jQuery Mobile), but the rest was pretty straight-forward. I had to defile your localStorage abstraction a little bit because I thought converting the string to a boolean in both model and view is not the right way to go.

Any feedback?

Cheers,
Alex