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)


Also, my reading of the discussion with @stefanv was that we would go with "labels" rather than "label_image", despite my own preference. @stefanv, I gave a bit more of a rationale for my preference of "label_image" — do you stick with your "labels" preference?