Stable (sample) variance

@sritchie: the code feels a bit messy due to my newness with clojure, cascalog, and midje-cascalog; I'm open to suggestions for getting this to merge-able condition.

This PR implements a parallel sample variance calculation that's numerically stable, following Gavin Heavyside's post at https://groups.google.com/forum/#!searchin/cascalog-user/variance/cascalog-user/N0E8FwCv9VU/1LV6kFiGxTwJ.

The test case comes from http://www.johndcook.com/standard_deviation.html.