Reported by Reiner Prokein
A .obj file can have multiple mtllib statements with the same name. Ensure that only one occurance is saved, so we don't get multiple entries from same material.
Also fix tabs -> spaces
- made operator dir's into python packages
- lazy loading of module which do the actual import and export (faster blender load times)
- general maintanance and small fixes.
- bugfix for exporting x3d materials
- leak fix for exporting 3ds