Revealed track image feature

I've added a property to the range slider.
It allows setting a track image that is revealed (cropped) rather than stretched.
This way the developer can set a background image that is not necessarily of one colour.
In addition, I've added an example to the demo application as you can see in the attached image.

![screen_shot_2013-11-18_at_11 57 08_am](https://f.cloud.github.com/assets/467722/1562189/13c542f4-5042-11e3-94dc-9c906eda822e.png)