Color samples by different columns in metadata

check this out - color the Parikshak 2013 data (derived from brainspan) by brain region:

![image](https://cloud.githubusercontent.com/assets/806256/5371659/1d830928-7ff0-11e4-8e66-257f0b7bc9c8.png)

And now color them by `SubjectID`, aka the donor ID

![image](https://cloud.githubusercontent.com/assets/806256/5371666/65847806-7ff0-11e4-9393-516f09fa5ed5.png)