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'd add an extra line here for readability:

```
default = {'weight': -np.inf}
w1 = g[n].get(src, default)['weight']
```
```