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
Hmm, ParseField.match nevers throws with empty flags, what is your goal here? Also, is it ok to ignore the return value of `match`?