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 think there is a thread safety issue here since this method updates an instance member while it could be called from separate threads at the same time? I think it's ok to allocate one each time as this would be an easy job for the JVM's escape analysis?