Use facecolor and edgecolor when saving figure in gen_rst

On a black background, the white colorbar ticks were not visible in the saved figure.

# Master

![without_facecolor](https://cloud.githubusercontent.com/assets/1680079/5246351/475fd64c-7968-11e4-8424-7c58517ba740.png)

# This PR

![with_facecolor](https://cloud.githubusercontent.com/assets/1680079/5246353/4b36e83c-7968-11e4-9073-e5fc65338db8.png)

Objections anyone?