RAG drawing function

![figure_1](https://cloud.githubusercontent.com/assets/3823490/3727712/e797aec6-16a0-11e4-9281-656fe0951a06.png)

![figure_2](https://cloud.githubusercontent.com/assets/3823490/3727719/f2e2a9a2-16a0-11e4-8667-bc06b7f59a9d.png)


Firstly, prefer iteration-based approaches to setup + increment. So here:

```python
for offset, (node, data) in rag.nodes_iter(data=True):
    # [...]
        rag_labels[labels == l] = offset + 1
```

But secondly, this approach will be ridiculously slow, because you are doing an O(`labels.size`) operation for each and every label. You should create a numpy array as a map and then do the transformation in one step. See the [`relabel_sequential`](https://github.com/scikit-image/scikit-image/blob/master/skimage/segmentation/_join.py#L55) function or [`Tree.get_map`](https://github.com/jni/viridis/blob/master/viridis/tree.py#L100) from my ultrametric tree library for examples of numpy-arrays-as-maps.

And thirdly, don't use `l` as a variable name, as it is sometimes hard to read and distinguish from 1. Prefer `label`. Or anything else. =P