Add compatibility for invididual celltypes

This enabled:

- Violin plots in celltype order
- Legends on PCA


Featurewise:
![image](https://cloud.githubusercontent.com/assets/806256/3697959/478435a0-13b5-11e4-89bf-6b978d95bb4c.png)

Samplewise:
![image](https://cloud.githubusercontent.com/assets/806256/3697983/c9a7e2f2-13b5-11e4-8002-3fd52593a032.png)


Also added, `BaseData.outliers` to do future separate analyses.