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)


There is *way* too much redundancy here. Do not repeat yourself. This function should be just:

```python
def construct_rag_meancolor_2d(img, labels):
    return contruct_rag_meancolor_3d(img[np.newaxis, ...], labels[np.newaxis, ...])
```

Better yet, the code should be n-dimensional to begin with.