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.
The formatting in this class isn’t consistent with other D3 methods. It should look like this:

```js
  var name1 = value1,
      name2 = value2,
      name3 = value;

  if (expression) return statement;
```