add nest.each function

I sometimes need a way to modify `{key: id , values: []}` nested objects. 

usage: 

```js
var yields = [
  {yield: 27.00, variety: "Manchuria", year: 1931, site: "University Farm"},
  {yield: 48.87, variety: "Manchuria", year: 1931, site: "Waseca"},
  {yield: 27.43, variety: "Manchuria", year: 1931, site: "Morris"}
];

var nest = d3.nest()
    .key(function(d) { return d.year; })
    .key(function(d) { return d.variety; })
    .each(function(o, depth) { o.text = "title: " + o.key; })
    .entries(yields);
```

Cheers
C.