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 don't feel strongly about *not* using that, but why do you say so? I think that it makes it less readable (this function name will appear in the docstring for `merge_nodes`) and that it's not crucial to keep this callback private. It might even be deleterious. For example, even though it is the default, some implementations might want to explicitly call it, for code clarity reasons.