feat(directives): Added two way databind for html5 contenteditable attri...

Hi guys,
This is a simple implementation for data bind between html5 contentenditable atributes. First I find this thread in stack overflow: http://stackoverflow.com/questions/15108602/two-way-binding-of-contenteditable-item-inside-ng-list 
It uses AngularJs.
My first try was translate the code in stack overflow to Dart. But after I read the code on _InputTextlikeDirective in the same file, I realize that it do more checks and make the same think, so I just override the get and set for the value be the innerHtml instead of value

I also open this thread for angular dart: http://stackoverflow.com/questions/21055252/how-to-enable-databind-in-contenteditable-on-angular-dart