Angular 1.2.x model binding support

Fixes an issue with model.$render appeared with angular 1.2.0-rc3 where the editor isn't updated when the model changes : https://github.com/angular/angular.js/issues/4560. This applies to ui-tinymce too.
Thanks to @iwang for the fix :
http://iwang.github.io/html/angular/angularjs/2013/11/04/ngmodel-render-cannot-be-overriden-in-angular-rc3.html