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.
I didn't spot the difference between the "group" and "object" and chose the 'g' notation because of many examples using it (and because each object is a group of others objects).

It's working with both 'g' and 'o', so the choice is yours ;)