SERVER-5044 stdDev agg opt for standard deviation

Working standard deviation and variance aggregation operators,
implementing algorithm from
https://gist.github.com/RedBeard0531/1886960.
Unit tests also added.

Added the following aggregation operators:
$stdDev - calculate the standard deviation of the sample
$var    - calculate the variance of the sample
$stdDevPop - calculate the standard deviation of the population
$varPop    - calculate the variance of the population

https://jira.mongodb.org/browse/SERVER-5044