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)


I'm not sure there's much to be done here to make it more intuitive: `edges_iter` returns an iterator over `x, y` (just the node IDs), unless `data=True` is passed, in which case, it returns an iterator over `x, y, data`, where `data` is the dictionary holding the edge attributes.