basic, ascii stl export works
- added basic, but working Amf exporter ( closes #69)
 * basic shape export works
 * metadata integration also partially there
 * basic (one level) hierarchies also in there
 * most of the components for colors , textures, materials are there but not hooked up to get their data from the three.js data structures
 * need to add unit tests (verbosity of the output is a bit problematic though)

- a bit of cleanup of stl exporter