OBJExporter: handle children objects

These improvements with the OBJExporter allow to export THREE.Object3D and their children.

+ add a *scaleFactor* param, to scale the final object.

BC break:
- OBJExporter need an Object3D (and not an object.geometry like the previous version).

It's my first PR, so feel free to give me your feedback.
Yeah, probably better to scale things before/outside of the exporter.