Added `spacing` to `d3.layout.pie`

This is very handy instead of using `stroke-width`, which diminishes the visual value of small arcs in a pie/donut chart, and might intersect with other shapes in the chart. The spacing that each arc gets does not count against its value.

Example:

```javascript
pie = d3.layout.pie()
        .spacing(2 * Math.PI * 0.01); //Each arc gets a spacer of 1% of the circle's size after it
```

![Example](http://i.imgur.com/jiUk48t.png)