Bind to ISO-3166 alpha-2 code instead of country name.

Hello!

I've just made a quick change to your directive to bind `ng-model` to the ISO-3166 alpha-2 country code instead of to country name, due to the way I'm storing countries in the database. It's not backwards compatible, but I guess you could simply modify the `ng-options` value in the directive to switch back. :+1: