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 guess it must come in somewhere that I'm not seeing because the tests pass. Could you add a comment about where that -1 is (so that future readers find it more easily)