Refactored stl.py into new: Model3D.py

This should make it easier in the future to add different file types,
as they can all just inherit from the 'abstract' class Model3D. All I
need to do now is move some of the processing away from the main
program and into a class/module of some sort.
Merge pull request #5 from matthewelse/refactor

Refactored stl.py into new: Model3D.py