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 don't think it is useful to repeat the description of the parameters since they should be inherited from the parent class. So maybe we should just remove the javadocs of this method and move the reference to the source of the algorithm to the class-level javadocs?