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 want users to be able to discover the features they need to know.  As long as it is clear that this is a networkx graph, then no problem (but don't expect them to read the source).

In my copy of NetworkX, the graph methods are not listed.