Add assimp / assimp2json importer

This is a follow-up to #3565, adding a assimp2json loader THREE's samples. [assimp2json](https://github.com/acgessler/assimp2json) is a simple JSON command-line converter for [Open Asset Import Library ](http://assimp.sf.net) (short assimp), which reads about 40 3D file formats, including 3ds, obj, dae, blend, fbx, x, ms3d, lwo, ifc, md3, stl ... . 

It adds the `webgl_loader_assimp2json` sample and the `examples/js/AssimpJSONLoader.js` importer implementation, as well as two sample scenes in `examples/models/assimp`. Those are from the assimp repository and compatible with THREE's license. One is self-made long time ago, the other had a note allowing for any redistribution.

I have not contributed to THREE before, let me know if formatting or code structure should be changed! :-) 

![assimp2json sample scene](http://s21.postimg.org/o6baspxmv/assimp2json_sample.png)