Add help text to single checkbox and radio

Help text is not rendered under single checkbox and single radio. Example form builder:

    $builder->add('gender', 'choice', array(
        'choices'   => array('m' => 'Male', 'f' => 'Female'),
        'multiple' => false,
        'expanded' => true,
        'required'  => true,
        'mapped' => false,
        'attr' => array(
            'help_text' => 'Who are you?'
        )
    ))
    ->add('newsletter', 'checkbox', array(
        'mapped' => false,
        'attr' => array(
            'align_with_widget' => true,
            'help_text' => 'Would you like to receive our newsletter?'
        )
    ))

renders:
![image](https://cloud.githubusercontent.com/assets/4865922/3604684/ce479c06-0d22-11e4-9276-993c1321c5d6.png)

With the patch:
![image](https://cloud.githubusercontent.com/assets/4865922/3604696/f806c44a-0d22-11e4-9f17-cc0251298aec.png)