Show values in `Doughnut Chart` and `Pie Chart`

`Pie#defaults` and `Doughnut#defaults` add some options

```js
scaleShowLabels: false,
scaleLabelPaddingX: 35,
scaleFontFamily : "'Arial'",
scaleFontSize : 12,
scaleFontStyle : "normal",
scaleFontColor : "#666",
scaleLabel : "<%=value%>",
...
```

![screen shot 2013-05-26 at 3 36 52 am](https://f.cloud.github.com/assets/27926/564511/831df46c-c572-11e2-965f-b27721c04a5e.png)
![screen shot 2013-05-26 at 3 36 41 am](https://f.cloud.github.com/assets/27926/564512/899415ec-c572-11e2-9b08-ea48ad5d826e.png)