Expose Asset id
I'm not really sure why [Asset](https://github.com/BohemianCoding/SketchAPI/blob/c8609b6180657e59bc48a139d6a4b8ec1f1ef45e/Source/dom/assets/Asset.js#L5) needs to remove the `id` property. `MSColorAsset` and related classes have an `objectID`. Would it be problematic to expose it?