Expose the original OBJ "illum" value

[TerenceRussell]

This commit exposes the original Obj model illum value (illumination_model) as a material property.

The illum value is defined in a .mtl material file like so:
illum 3

Background:
ObjFileImporter::createMaterials in ObjFileImporter.cpp partially interprets the illum value as an aiShadingMode enum and stores it in the AI_MATKEY_SHADING_MODEL material property. Illumination model values 3 thru 10 are lost in the interpretation. Exposing the original illum value allows user interpretation of the intended model.

Example accessor usage:
aiScene* scene = ...
int illum = 1;
aiMaterial* mtl = scene->mMaterials[m];
aiGetMaterialInteger(mtl, AI_MATKEY_OBJ_ILLUM, &illum);

[kimkulling]

Looks good.

[kimkulling]

Mered, thanks a lot for your contribution.