add support for normal map loading.
attach material properties to points and lines as well.
add support to load diffuse color texture maps.
don't fail if there's no material file.
don't try to find a material when the list is empty.
added support for material (.mtl) files.
allow object names to have '.' and '_'.
improve element insertion procedure.
add support for multiple vertex type indices in face elements.
removed unnecessary anonymous union in OBJVertex.