Allow full-screening an element by watching a $scope property

This allows any element to go into full-screen mode without having to use IDs. Simply toggle a property on the `$scope` and pass in the property's name to the directive.

This eliminates the need to rely on IDs, and eliminates the need for the "bad practice" approach described in the README.

Note: This pull request is dependent on https://github.com/fabiobiondi/angular-fullscreen/pull/1