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? 
I think this should be:
`Yields `nan` if both of the decreased counts are 0`