Refactored SU scene representation, basic exporter.

The SU scene representation has been refactored. The most important change
is renaming 'Object3d' to 'Part' to avoid name ambiguity between 'object'
as a 3-D object and as an entity of OO language.

Basic exporter has been added. Still, there is no UI (the exporter can be
run from the Ruby console).