expr: tukey{Above,Below} support for negative intervals
expr: tukey{Above,Below} support for negative intervals

A negative interval will be defined to perform the tukey test over all
*except* the most recent abs(interval) points. Eg.
    tukeyAbove(the.metric.*,1.5,5,"-2min")&from=2h
would perform the test for points from 2 hours ago until 2 minutes ago.

errx/carbonapi,arodland/carbonapi,errx/carbonapi,dgryski/carbonapi,nnuss/carbonapi