Adds 'form-control' class to the dropdown-toggle button

I need to highlight the control in a validation. Bootstap do it if the control has the "form-control" class.

![has-error](http://i.imgur.com/t1jHa5v.png)

Maybe there are many other solution, like the chance to define custom classes in the initialization as an option. But, since this is a Bootstrap aware plugin and the control is going to be a part of a form, I think that this solution is simple and could be included.