Support of polygons with interior holes

[eshira]

Request for support of polygons with interior holes. I think the core of the code will work as is if there is some way to represent them.

For now my hack is this: when creating the list of polygons for the graph creation, specify the exterior polygon ring first, then put a blank polygon (empty list) in front of every matching interior ring. Then in the init method for the graph I catch those empty lists and rewind the pid by one. The resulting behavior for the visibility graph seems fine so far.