Added radial gradient handling.

For presentation purposes we needed a radial gradient for a shape to allow a written text over the shape to be readable independent of the color of the shape. An example result is shown in the image below.
![image](https://cloud.githubusercontent.com/assets/7868114/4057765/28e5f3ae-2dcd-11e4-8965-7868575fc567.png)

Could you pull this change?
Thanks.
Best regards, 

Michiel