Added Polygon class

Me, being a Student of the FAU Erlangen-Nuremberg, used the Boxmaker a lot to Generate boxes for our Laser Cutter at the FabLab. But one thing always bothered me: You can't easily edit the files, as the parts weren't pdf polygons, they are a lot of little lines. This is not only a problem with editing, but also with cutting. Our laser didn't cut the parts as one piece, but rather cut every single line, then jumped to a different line, etc.
So I wrote a Polygon Class that automatically merges all lines to polygons.

I didn't change anything at the actual code of the box generation, so, everything should still be as it was before. Just better.

Hope the help is appreciated.
Greetz,
Martin