Add gulp-order and configure it

Here is an improvement to the javascript gulping.

**Background:** I wanted to create a global object `demokratikollen` in a Javascript, so we can access common functionality with expressions like `demokratikollen.utils.colors` or `demokratikollen.graphics.AppointmentsTimeline()`. These scripts should be concatenated and minified with the other javascripts we have started collecting in `demokratikollen/www/design/js`, but since some of them depend on others, we have to make sure they are concatenated in the right order.

**Solution:** The `gulp` plugin `gulp-order` (https://github.com/sirlantis/gulp-order) allows a reordering of the incoming files. This pull request contains the installation of `gulp-order` and a basic setup in the gulpfile that should be straightforward to extend as you add more js files.