Allow other attributes and additional classes on tab-panes

If you add attributes like so:

    $tab = $builder->create('mytab', 'tab', array(
        'attr' => array(
            'class' => 'something',
            'data-id' => 45,
        ),
    ));

It will show up in tab panes:

    <div class="tab-pane active something" data-id="45">