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)


Hrm, rethinking this I would prefer to keep the object lightweight, because some operations may make it unrelated to the original labels.  Then, one would have to update the labels too, and not sure we want to go that way.