Fixed a bug returning not all material properties when accessing them.

Also fixed another bug occouring when no normals are provided in sample.py (lists do not have a any() method). And added mesh triangulation to sample.py, otherwise PyAssimp is even not able to load "untriangulated" meshes.

The property solution is not the best way to do it but I don't want to screw the API. A better solution could be a Property class which has the additional semantic attribute stored and queries for attributes where multiple semantics exist should return dicts in the form of {semantic: property}.