Fix issue #6: No robot model when using display.launch (again)

This pull request should actually fix the issues. I think the proposed file and directory layout is in line with other packages containing URDFs and launch files.

Doing it this way does require us updating the tutorials, as the URDFs are now in the `urdf` subdirectory.

PS: I left the `urdf.vcg` file in, although AFAIK those are not used anymore (old RViz config file?).