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.

This makes sense if you e.g. want to add the sd to a chart or calculate a gaussian standard deviation curve.