stats: Fix computation of summed standard deviation

stats: Fix computation of summed standard deviation

Fix the computation of standard deviation for a group
of jobs. Please see the below link for the
approximation formula used.
<http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance

The formula was originally presented here:
<ftp://reports.stanford.edu/pub/cstr/reports/cs/tr/79/773/
CS-TR-79-773.pdf>

Signed-off-by: Yu-ju Hong <yjhong@google.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>