Add a length class to breadcrumb

Using SonataUserBundle, i noticed the breadcrumb wasn't full length so it would be useful to add a col-md-12 on it.

Without it the breadcrumb bar becomes taller and goes over the content block like in the screenshot : 

Before :
![image](https://cloud.githubusercontent.com/assets/3241292/2611834/016705d8-bbaf-11e3-9ee0-ecc6fb9c3ba5.png)

After : 
![image](https://cloud.githubusercontent.com/assets/3241292/2611841/2ecd51d0-bbaf-11e3-871a-58b3201245ff.png)