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)


This only happens when we use `regionprops` and want to measure properties of regions. I think the correct place to fix this would be in the segmentation module. If we have regions labeled `0` they will always be ignored by `regionprops `