Sends width and height as another parameter to the callback. Fixes #67

This adds a 2nd or 3rd parameter to the callback which looks like this:
```
{     
     width: 1920,
     height: 1080
}
```

I'm not sure if I did anything wrong as this is the first time I've written any node C++ code. Also added an additional parameter to the promise call, which again I'm not sure is right. In any case it's working nicely.