Artboard object has no export method
Hello! So on, I have been working on my plugin for a while, and I have experienced the issue with exporting artboards.

I have an array of duplicated artboards:
```js
var tmpArtboards = [];
rootLayers.forEach(function(rootLayer) {
    var tmp = rootLayer.duplicate();
    // ...
    tmpArtboards.push(tmp);
});
```

I try to set the export options for the artboards:
```js
var options = {'scales': '1', 'formats': 'png'};
tmpArtboards.forEach(function(artboard) {
    artboard.export(options);
});
```
I get this:
```
TypeError: artboard.export is not a function. (In 'artboard.export(options)', 'artboard.export' is undefined)
```
As I know, **Artboard** inherits **Layer** and I should be able to use `export` method, but it seems that these objects are not belong to **Layer**.