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 was considering it but I couldn't easily find a paper on it, so I went with the simplest adaptation of the Chan paper that I could come up with.  It doesn't mean that it is right, but I can't tell which would be better without something to compare.

What would you think if (just for testing) I wrote a version using float32 instead of float64 to get a sense for the error?