Recommended improvement for the Object/MTL-Exporter

It is more common to use the following version for the definition of the material colors and texture maps in the *.mtl: 
The first character should be upper case (for example `Kd` instead of `kd` or `map_Kd` instead of `map_kd`). Some tools (e.g. Meshlab) only accept this version. It would increase the compatibility to other tools if you change this.