[Form] addAttribute function

Often have to do so:
<pre>
if ($form->hasAttribute('class')) {
    $form->setAttribute('class', $form->getAttribute('class') . ' form-horizontal');
} else {
    $form->setAttribute('class', 'form-horizontal');
}
</pre>
But it is much easier:
<pre>
$form->addAttribute('class', 'form-horizontal');
</pre>