Issue/#26

Fixed #26 
```Adapt.device.screenSize``` have three value ``` 'small', 'medium' and 'large' ```.
As we have removed the ```'medium'``` attribute from ```example.json```, so if the screen size is ```'medium'``` than ```this.model.get('_graphic')[width]``` would be ```undefined```.

So i could suggest one more variation
```
if (width === 'medium') {
    this.$('.graphic-widget img').attr('src', this.model.get('_graphic').small);
} else {
    this.$('.graphic-widget img').attr('src', this.model.get('_graphic')[width]);
}
```

what do you think?