Fix standard deviation formula

The standard deviation formula is sqrt(deviations / (N-1)) but
not sqrt(deviations/N - 1). I'm not sure but probably standard
deviation must be 0 when N==1. In any case we can't divide by
zero.

Signed-off-by: Serj Kalichev <serj.kalichev@gmail.com>
Merge pull request #2 from pkun/std_deviation

Fix standard deviation formula