Add google normalized distance and chi square to significant terms

For google normalized distance, see
"The Google Similarity Distance", Cilibrasi and Vitanyi, 2007
http://arxiv.org/pdf/cs/0412098v3.pdf

For chi square, see
"Information Retrieval", Manning et al., Eq. 13.19
I use the equals(..) in tests in SignificanceHeuristicTests . I thought (and @s1monw requested in [this comment](https://github.com/elasticsearch/elasticsearch/pull/6561#discussion_r14647971)) if you override the equals you will also have to override hash just to be on the save side. 
So, now I added a hash function for each of the heuristics to avoid collisions in commit  "add hashCode() to each heuristic that actually has instances". Please check if this is the way to write a hash function.