In order to access gridstack's API within AngularJS application. We need to expose `gridstack` object which was created in `GridstackController`.

**Usage:**

When init the HTML template for gridstack:

```html
<div gridstack gridstack-handler="gridstacker" options="options">
   ...
</div>
```

In your AngularJS application:

```js
$scope.gridstacker = null;
```

Don't forget to check for `gridstacker` availability before using:

```js
if ($scope.gridstacker) {
    $scope.gridstacker.removeAll(); // example: call removeAll() API
}
```

Expose gridstack object to Directive

…d dist files