PieChart transparent hole

First of all, thanks a lot for your awesome works on charts!

Here is a small tweak I add to PieChart in order to enable transparent hole. 

In my case, I used a textured background that's why I decided to add this feature.

![piechart](https://cloud.githubusercontent.com/assets/4211470/4919331/55f24d3a-64f9-11e4-8a15-26f3f08d1e4d.png)