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)


Hmm... Probably one of these "outdated diff" comments but I can't find it. Anyway, the basic idea is that `labels` might be more suitable to the complete list of labels, ie:

```python
labels = np.unique(label_image)
```

This is useful in many contexts. The alternative would be:

```python
unique_labels = np.unique(labels)
```
What do you think?