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
These checks seem to be duplicated in most instances, can we try to share the code in a parent class somehow?

Additionally, in that particular class, it seems like 0 should be avoided as well since we are taking a log?