Forbid scalar values on $scope

[tilmanschweitzer]

From the angular wiki (https://github.com/angular/angular.js/wiki/Anti-Patterns):

Don't use a scalar variable (null is scalar) as a model within an isolate scope (such as ng-if). (Example: http://embed.plnkr.co/qRhLfw/preview)

(Last missing point from #133)