Buttons can be dismiss buttons

There's `Close` but buttons can also be dismiss buttons for `Modal` or `Alert`, see http://twitter.github.io/bootstrap/javascript.html#modals. I've added a `setDismiss()` method.

UiBinder example

```xml
<b:Modal>
    <b:ModalFooter>
        <b:Button text="Close" dismiss="MODAL"/>
    </b:ModalFooter>
</b:Modal>
```