Make sure objects are all visible
Allow to give a path as arg, create target dir if needed

While here, changed default path for exported meshes to ./mesh/stl
Automatically quit Blender once meshes are exported for cmd line invokation

The conversion can now be triggered with:
$ blender nao-v4.blend -P io_export_separate.py
or
$ blender nao-v4.blend -P io_export_separate.py -- --path=/my/path/