Add support for PNG image output.
Fix bugs in pbrt code generated by obj2pbrt when textures are present
Fix more bugs in obj2pbrt

Handle more types of bump map.
Parse bump map scale factor
Issue warnings on ignored bump map options
Don't scale image textures by (0,0,0) if the user hasn't specified an
  actual Kd (etc.) scale factor for the corresponding component.
Bump copyright year.
Print more informative error message for unsupported image file formats.
Merge remote-tracking branch 'upstream/master'