add bootstrap-popover as display option for help

Like in #401 I'd like to use a popover (http://twitter.github.io/bootstrap/javascript.html#popovers) for my help text, as I need some well formatted title/content including some HTML-formatting.

Usage:

```php
$builder->add('field_name', 'field_type', array(
  'help_label_popover'  => array(
    'title' => 'my helpful popover title',
    'content' => 'my helpful popover content',
    'icon' => 'css class of my shiny help icon', // e.g. bootstrap 'icon-info-sign'
    'placement' => 'top' // how to position the tooltip - top | bottom | left | right
  )
));
```