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)


This block seems a bit clunky to me. Here's what I've come up with:

```python
if current not in g:
    g.add_node(current, {'pixel count': 0,
                         'total color': np.zeros((3,), np.long),
                         'labels': [current]})
node = g.node[current]
node['pixel count'] += 1
node['total color'] += image[iter.multi_index]
```