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 ambivalent. =) A reference is cheap enough but it adds a bit of complexity. However, I think we can safely assume that we'll need it at some point, so it's probably a good idea. @stefanv thoughts?