Added Variance and Corrected Standard Deviation to core array functions

D3 already has functions to get the `max`, `min`, `mean` etc. of an array. What is missing is the calculation of the variance and standard deviation. For that I needed an other library like jStats, but I think it's very handy to have them available right from D3.

    d3.sd([1,2,3,4,5]) => 1.5811388300841898
    d3.variance([1,2,3,4,5]) => 2.5

This makes sense if you e.g. want to add the sd to a chart or calculate a gaussian standard deviation curve.
Allocating another array and then calling d3.variance recursively is slow, and takes about the same amount of code as just computing the variance directly on the evaluated values, which is what we should be doing here. (See other stats functions for examples.)