feat($rootScope): adds $onRootScope method

This adds an $onRootScope method to the
scope type. This allows developers to use
$rootScope.$emit + $scope.$onRootScope as
a fast eventBus which doesn't use scope
bubbleing.

Fixes #4574, Relates to #5371
shouldn't we unsubscribe both listeners? `root.$on` & `this.$on`