make ItemSelection react on route.url changes

This is a popular problem:
https://github.com/ember-addons/bootstrap-for-ember/issues/62
http://stackoverflow.com/questions/14328295/how-do-i-bind-to-the-active-class-of-a-link-using-the-new-ember-router
http://stackoverflow.com/questions/15556897/getting-the-active-route-and-applying-it-to-a-nav-in-a-view
http://stackoverflow.com/questions/14412073/assigning-active-class-to-selected-list-item-in-emberjs

If you accept it then my previous pull request https://github.com/ember-addons/bootstrap-for-ember/pull/66 is not actual