- modfify straight skeleton to work around bug in CGAL with polygons
  with touching holes
- added polygon with hole and polygon with touching holes in
  StraightSkeletonTest
- removed leak (bug maybe introduced intermittent bug) in
  IntersectionTest
- fixed all warning (now compiles with -Werror)
- changed operator = to copy&swap idiom (avoid risky/buggy copy to self)
all tests pass
- fixed warnings in release