Added support for ngModel to be used seemless with isolate scope

Fix for https://github.com/angular/angular.js/issues/1924 

Added implementation + Updated documentation.

You no longer need to do 
```html
<my-component ng-model="$parent.my.model"></my-component>
```
And can instead write directives using isolate scope that behave as you would expect: 
```html
<my-component ng-model="my.model"></my-component>
```
Additional discussion: https://github.com/angular/angular.js/issues/1924#issuecomment-20547402