Blender Git Loki

Export UV vertex coordinates & their indices.

This commit adds support for UV vertex coordinates in the form:
`vt u v` for all vertices in the texture map.

Also, their indices are exported as per file format specification [1,2]
`f */vt1/* */vt2/*` for all polygons.

The text written by this and the python exporter has different order
of coordinates and thus their indices too. So direct diff of the two
files will not work.

Minor comments about 0-based/ 1-based indices are also added.

[1]: https://en.wikipedia.org/wiki/Wavefront_.obj_file#File_format
[2]: http://www.martinreddy.net/gfx/3d/OBJ.spec