Dysbiosis index

Addition of the microbial dysbiosis index described in [Gevers et al 2014](http://www.ncbi.nlm.nih.gov/pubmed/24629344). This method has been generalized to support other indices in the future as well.

@ElDeveloper, can you review please since you implemented the original approach? 
It's still correct but the indentation of this and the next option is way different than for the rest of the other options. Might be worth it to reconsider and make them all look the same.