Change menu class on custom menu widget

Hi

I am fairly new to wordpress but I am getting use to it. I have created my first theme and have been able to add custom menus into the theme no problem.

I want to have a menu that is only visible on some pages and I know that I can do that with the widgets adding a custom menu which I have done. And I know that I could use CSS to style but I am using bootstrap and would love to be able to change the menu class (currently <ul class=”menu”> to “nav nav-pills nav-stacked” – is this possible using a widget?

Thanks
Adam