Improved shortest path searches

Hi all, Thanks to a contribution by Zachary Pincus and Almar Klein, we now have much improved shortest path (least cost path) algorithms in scikits.image.graph, based on binary heaps. Feel free to review and play around: http://github.com/stefanv/scikits.image/tree/master/scikits/image/graph/ Cheers Stéfan