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)


```python
from skimage.graph import rag
help(rag.RAG)
```
This does print out a section saying, `Methods inherited from.......`
Although doing `rag.RAG?` in IPython does very little, but then again, so does doing `BivariateSpline?`