Merge pull request #17 from iRi-E/master

PMX importer: Sphere map support and fixes for material setup.
Cycles converter: To simplify, use material.texture_slots[j].use rather than material.use_textures[j].
Render setup: Use display device "None" in the "Shadeless" settings.  Also change the default value of spa_blend_factor option in PMX importer to 1.0.