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
The class of the object is not taken into account so it would be very easy to generate collisions with instances of MutualInformation and ChiSquare. Do we use these objects as keys of hash tables or rely on the equals method somewhere?