feat(form): publish NgForm instances in the context

The forms are now available in the context field named after the form:

    @Component(
        selector: 'my-component',
        template: '''
          <form name="myForm">
            <input type="text" ng-model="model" name="input">
          </form>
        '''
    )
    class Component {
      // myForm is automatically initialized to the NgForm instance
      NgForm myForm;
    }

When the field named after the form is not present, an exception is
thrown.