Add a labelType method for pieCharts

This pull request adds a `labelType` method for `pieChart`s to give to option to show the 'key', 'value', or 'percent' on the slice's label. With this change, using `.showLabels(true)` still defaults to this:

![screen shot 2013-08-07 at 8-7-13 10 36 07 pm](https://f.cloud.github.com/assets/1975147/929195/632fcf7c-ffd3-11e2-8ac2-23ae8757c024.png)

but now you have the option to use `.labelType("percent")` to show this:

![screen shot 2013-08-07 at 8-7-13 10 36 24 pm](https://f.cloud.github.com/assets/1975147/929198/82c319a2-ffd3-11e2-8e80-4fd5da33f4f3.png)

Open to feedback. Let me know what you think.