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)


`draw:single_colormap` will draw with just one color, right ?

How about we add a `color` argument, which will draw all edges with a single color and `colormap` argument which taken any matplotlib colormap ?