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.
`object.children` is always available, no need to check.