Added Variance and Corrected Standard Deviation to core array functions.
renamed sd to deviation #1073
added exit criteria for variance that returns NaN on an empty array
refactored results with NaN array and empty arrays to fit the results of R
using an evaulation array for accessor functions and ignoring NaN, undefined and null
passing f to d3.mean function call in the variance implementation
reimplemented variance to be calculated incremental using Knuth´s and Welford´s approach
returning `undefined` rather than `NaN` to comply with d3.mean as @mbostock suggested
retabbed with expandtab and tabstop=2
Merge branch 'master' of github.com:southdesign/d3 into deviation