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.
Sorry, my comment below before was meant to change the documentation comment. It's probably also okay to just remove the sum_j {[w_j} -1 bit.