refs #318 fix zIndex of valueThumb and extentThumb

Before:
![image](https://cloud.githubusercontent.com/assets/285338/5065287/feb9f36e-6e5e-11e4-8c2a-2317e5f03fb0.png)

After:
![image](https://cloud.githubusercontent.com/assets/285338/5065283/dbd034f8-6e5e-11e4-9ea9-3fed42a9ad92.png)