feat(ngForm): Supports expressions in form name

Now it is possible to access a form from a controller using the new `controller as` syntax, without having to access `$scope`:

Old way:
```html
<div ng-controller="MainCtrl as main">
  <form name="form" ng-submit="main.submit()">
  </form>
</div>
```
...
```javascript
.controller('MainCtrl', function($scope) {
  this.submit = function() {
    if $scope.form.$valid {
      // do something!
    }
  }
})
```

New way:
```html
<div ng-controller="MainCtrl as main">
  <form name="main.form" ng-submit="main.submit()">
  </form>
</div>
```
...
```javascript
.controller('MainCtrl', function() {
  this.submit = function() {
    if this.form.$valid {
      // do something!
    }
  }
})
```