Working edition. Some problems generating the skeleton for angles < 80
between to edges. Implemented by paper algorithm (Skeleton Extraction of
3D Objects with Visible Repulsive Force, Fu-Che Wu, Wan-Chun Ma,
Ping-Chou Liou, Rung-Huei Liang and Ming Ouhyoung). Soon bugs will be
fixed, docs will be added and 3D version. Geometry library by Mikhail
Dubov.