Make golint happy. Change StDev to StdDev.

This CL changes the function signature from StDev to StdDev to match the other StandardXxx functions.
Additionally, this PR fixes the comments that don't start with the correct name
and changes +=1 to ++ by recommendation from golint
Merge pull request #2 from gonum/bug-golint

Make golint happy. Change StDev to StdDev.