Button: Remove some style options form the widget.

Hi all 

I add css rules for negating a style option.

 - data-corner
 - data-shadow

![button-negate](https://cloud.githubusercontent.com/assets/3013966/2796603/0a7bfec6-cc12-11e3-9f30-6d0b1cdcce86.png)


If there are any problems or mistakes on that PR, please let me know. Thanks in advance.

Fixes gh-7303
Ref #7304