Bootstrap 3 multiselect plugin as form element

I'm trying to add multiselect plugin to my form. I've managed to get everything working just fine (http://jsfiddle.net/Misiu/t2qaP/), but I would like to get that select to be 100% width.
I can add css by hand:

 .btn-group, .btn-group .multiselect {
    width: 100% !important;
}

but maybe there is better way of doing this with bootstrap?

here is my demo: http://jsfiddle.net/Misiu/t2qaP/