Include how many members are in the percentile threshold.
Add tests to verify percentile counts.
Add "sum of squares" to the pre-calculated timer metrics.

Sum of squares (along with count and sum) can be used to do rapid
calculation of standard deviation/variance over an arbitrary number
of consecutive intervals [1].

[1]: http://en.wikipedia.org/wiki/Standard_deviation#Rapid_calculation_methods
Merge pull request #376 from mheffner/feature/more_counter_stats

Add counter and sum of squares to timer stats