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.
d3.mean checks arguments.length, so passing in `f` it is undefined will crash. (Which is surprising since you appear to have tests which cover this.)