WebButtonGroup doesn't group buttons if added with 'add()'

This will group the buttons:

``` java
WebButtonGroup group = new WebButtonGroup(WebButtonGroup.VERTICAL, true, button1, button2, button3);
```

This won't group them:

``` java
WebButtonGroup group = new WebButtonGroup(WebButtonGroup.VERTICAL, true);
group.add(button1);
group.add(button2);
group.add(button3);
```
