change the ngModel.$render to scope.$watch because ngModel.$render did n...

In this plunker: http://plnkr.co/edit/SMl3pqLP0XF7Bm4FM5Op?p=preview I explain the problem that this pullrequest fix. There, we have a text input and a tinymce editor with the same model, which is $scope.tinymce.
If you change the model in tinymce editor, the text input value changes. If you change the model in text input, the tinymce editor doesn't change.
Here is a plunker with the new code: http://plnkr.co/edit/4LO1EOqz48XiAa1eS7BI?p=preview