Add counter and sum of squares to timer stats

This adds the count of members in each percentile and the sum of squares to the precalculated timer statistics. These are both used to do rapid calculation of standard deviation/variance over an arbitrary number of consecutive intervals: http://en.wikipedia.org/wiki/Standard_deviation#Rapid_calculation_methods.

This is a superset of the patch in https://github.com/etsy/statsd/pull/342

These pre-calculated values can be leveraged by the Librato backend to reduce duplicated timer processing.