change dropdown visiblity control to match bootstrap's

Bootstrap uses `display:none` to control visibility of a dropdown, changing it to `display:block` when `open`.

Flat-UI uses `opacity:0` and `visibility:none`.

The result is the same, but when you use a 3rd party bootstrap wrapper (eg [ui-bootstrap](http://angular-ui.github.io/bootstrap/)), these differences break the dropdown in that it will be `open` but, but will still have `opacity:0` and `visibility:none`.

Changing Flat-UI to use `display:none` fixes this.