add bootstrap-tooltip as display option for help

add tooltip which depends on bootstrap-tooltip.js (http://twitter.github.com/bootstrap/javascript.html#tooltips)

Usage:

```
$builder->add('field_name', 'field_type', array(
  'help_label_tooltip'  => array(
    'title' => 'my helpful tooltip',
    '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
  )
));
```