Simplify sigs

As discussed at https://groups.google.com/forum/#!topic/gonum-dev/nOWoTO2ilVM

This is more of a request for comment than something I want to merge right away.  In particular, I've changed some of the algorithms to use a correction.
I believe the conclusions here:
http://www.johndcook.com/blog/2008/11/05/how-to-calculate-pearson-correlation-accurately/

If we compute the mean and standard deviation for both first with a call to MeanStd then we can do a final 1 pass. This will be much more numerically robust.