add support for SVG for star rating sprites

before
![before](https://cloud.githubusercontent.com/assets/1492162/3202124/f4b7f21e-ed8d-11e3-82f0-0234e9b627a7.png)
after
![after](https://cloud.githubusercontent.com/assets/1492162/3202125/f871b26e-ed8d-11e3-94a8-b54fe4bdd6d6.png)

![](http://media1.giphy.com/media/lLkKpUBx8K6be/200.gif)