Updated stl.py

Moved facet processing to the surface class, instead of having to run
it in the base script.
Removed unnecessary parameter in stats() method.
Added SVG Export to pySlice.py

The main python script now outputs all of the pairs of points as lines
in an SVG file. This is still incomplete, as:
We don't have Python 3 support
It doesn't take into account triangles that lie on a plane #3 (all the
same Z coordinates), so that is why we still end up with missing gaps.
This is getting towards fixing #2