title: add class atribute to symfony form builder row div
body: <p>here is my AbstractType code :</p>
  
  <pre><code>   $builder-&gt;add('email', 'email', array('label' =&gt; 'form.email', 'translation_domain' =&gt; 'FOSUserBundle'))
  </code></pre>
  
  <p>code above generated :</p>
  
  <pre><code>    &lt;form class="fos_user_registration_register form-horizontal" role="form" method="POST" action="/app_dev.php/register/"&gt;
  
  &lt;div id="fos_user_registration_form"&gt;
   &lt;div&gt;
     &lt;label class="required" for="fos_user_registration_form_email"&gt;Email: &lt;/label&gt;
     &lt;input id="fos_user_registration_form_email" type="email" required="required" name="fos_user_registration_form[email]"&gt;
   &lt;/div&gt;
  &lt;/div&gt;
  
  &lt;/form&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>my question is how to add class attribute to the row div become :</p>
  
  <pre><code>&lt;div class="form-group"&gt;
      &lt;label class="required" for="fos_user_registration_form_email"&gt;Email: &lt;/label&gt;
      &lt;input id="fos_user_registration_form_email" type="email" required="required" name="fos_user_registration_form[email]"&gt;
  &lt;/div&gt;
  </code></pre>