Use the sample version of standard deviation (#383)

* remove unnecessary weights

* use sample standard deviation

* add contributor information

* remove redundant code

* initialize variable to eliminate compiler warning

CONTRIBUTORS

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 8ca4565..c0c0292 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -50,6 +50,7 @@
 Radoslav Yovchev <radoslav.tm@gmail.com>
 Ray Glover <ray.glover@uk.ibm.com>
 Shuo Chen <chenshuo@chenshuo.com>
+Yixuan Qiu <yixuanq@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
 Tobias Ulvgård <tobias.ulvgard@dirac.se>
 Zbigniew Skowron <zbychs@gmail.com>