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)


E.g., we can have the user-provided signature be:

```
merge(src_weight=None, target_weight=None)
```

Perhaps you'd like it to be more generic to access other node attributes?