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.
Everytime I try an export from my three.js object to a .obj, everything is way too big. I though it would be nice to have a scaleFactor param to easily scale the whole object. It's a faculative param (default to 1), but I understand it's not the goal of the OBJExporter to scale the object before export, so I can remove it.