title: Unable to extract translation id for form label with JMS Translation Bundle
body: <p>I'm using the JMS Translation Bundle to extract my translations. Everything works, but when I try to extract my translation messages form a form builder I recieve following message:</p>
  
  <pre><code>[JMS\TranslationBundle\Exception\RuntimeException]
    Unable to extract translation id for form label from non-string values, but got "PHPParser_Node_Expr_MethodCall" in /srv/local.project.com/app/../src/Project/MyBundleBundle/Form/Type/EmailType
    .php on line 30. Please refactor your code to pass a string, or add "/** @Ignore */".
  </code></pre>
  
  <p>The code I use for it is as followed:</p>
  
  <pre><code>public function __construct($translator)
      {
          //translation service passed from controller
          $this-&gt;trans = $translator;
      }
  
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder-&gt;add('email', 'repeated', array(
                  'type' =&gt; 'email',
                  'first_name' =&gt; 'email',
                  'second_name' =&gt; 'email-repeat',
                   //TRANSLATION GIVES ERROR ON NEXT LINE          
                  'invalid_message' =&gt; $this-&gt;trans-&gt;trans('online.form.email.errors.equal', array(), 'messages_roadrunner'), 
                  'error_bubbling' =&gt; true,
                  'first_options'  =&gt; array('label' =&gt; $this-&gt;trans-&gt;trans("online.form.email.fields.one", array(), 'messages_roadrunner')),
                  'second_options' =&gt; array('label' =&gt; $this-&gt;trans-&gt;trans("online.form.email.fields.second", array(), 'messages_roadrunner')),
              )
          );
  
          $builder-&gt;add('optin', 'checkbox', array(
              'required'  =&gt; false,
              'label' =&gt; $this-&gt;trans-&gt;trans('online.form.email.fields.optin', array(), 'messages_roadrunner')
          ));
  
      }
  </code></pre>
  
  <p>The strange thing is that in my controller I use the same thing, but there everything works as normal:</p>
  
  <pre><code>$content['text'] = $this-&gt;get('translator')-&gt;trans('online.tariff.text.no_advice', array(), 'messages_roadrunner');
  </code></pre>
  
  <p>Does anyone have any experience with this problem?</p>
  
  <p>Thanks in advance!</p>