Better stats

Added new statistical data to the StatsMetricService in a way that is "compaction" safe.  Added mean, variance (population and sample), and standard deviation (population and sample).  The default implementation uses the population variance and standard deviation as it is assumed that the metric data does not represent a sample population, but this is configurable if it does.

These are important values for allowing further statistical calculations such as finding probabilities based on a normal distribution which require the mean, and standard deviation.

This is in reference to Issue #46.