Adding in clustering and centroid labeling

[mortonjt]

This is related to #356 and discussed this with @cuttlefishh

It would be really nice if we could perform clustering and be able to label entire metadata categories with centroid labels.

For instance if we were looking at multiple studies, we could label each group of points that belonged to a specific study

[antgonza]

Guess that this is also related to: https://github.com/biocore/emperor/issues/18