Use 'box-sizing: border-box;' instead of 'content-box' for compatibility with Twitter Bootstrap

I include the bootstrap.css in my HTML page, and this bootstrap css uses 'border-box' for all elements: 

```
* {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}
```
And that conflicts with the 'content-box' sizing of the Office-Ribbon-2010, especially on using the 'ribbon-button-small' class.