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)


Actually, if the unique labels are not continuous, this will produce too many nodes. Perhaps it's best to move this below the call to `generic_filter`, and use:

```
for n in g:
    g.node[n].update({'labels': [i],
                      'pixel count': 0,
                      'total color': np.array([0., 0., 0.])})
```