Clustermap nested side colors

Added functionality to allow for nested side colors, for figures like this:

![image](https://cloud.githubusercontent.com/assets/806256/5134388/9a054dea-70c2-11e4-8f10-60546bec6906.png)

There's a test too :)
Let me know if there's anything else I can do!
I'm a little confused about why both of these are needed since they seem to be doing the same thing.