Add axiomatic similarity approaches to the similarity family.
More details can be found at http://dl.acm.org/citation.cfm?id=1076116 and https://www.eecis.udel.edu/~hfang/pubs/sigir05-axiom.pdf
There are in total six similarity models. All of them are based on BM25, Pivoted Document Length Normalization or Language Model with Dirichlet prior.
We think it is worthy to add the models as part of Lucene.