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)


Why not follow the Python convention for dictionaries?  I.e., something similar to d.keys() vs d.values() (you'll have to choose different names, of course).

Perhaps something like: ``rag.edges`` vs ``rag.weights`` or ``rag.edge_weights``.