[MRG] root2matplotlib: contour plots from Hist2D

![figure_1](https://f.cloud.github.com/assets/202816/1144601/b66e1a90-1db0-11e3-9728-2ebb59913014.png)

Sure beats the hacks required in ROOT just to modify the contour lines:

https://github.com/root-mirror/root/blob/master/tutorials/hist/ContourList.C