Region Adjacency Graph

## Example Images
![figure_1](https://cloud.githubusercontent.com/assets/3823490/3291718/5ce9b952-f587-11e3-8c29-e2aace6d7794.png)
![figure_2](https://cloud.githubusercontent.com/assets/3823490/3291723/7074b134-f587-11e3-8c73-d28222891a21.png)


@stefanv I kinda like this line! Unfortunately, there's no guarantee that either `src` or `dst` will have an edge to `n` (though one of them will). We could be more explicit (`if g.has_edge(src, n): w1 = g[n][src]['weight']; else: w1 = -np.inf`), but I think that ends up being much clunkier...!