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)


@vighneshbirodkar does the output need to be a float? Can we provide `out=np.zeros(labels.shape, dtype=np.uint8)` and return just `0`? That could save us some memory, but it could be that scipy requires the output to be of float type...