Adding 'help' as reserved key for FieldDescription

[stefanosala]

I added help to 'setOptions' to make it work with Admin as this example:

{
    public function configureFormFields(FormMapper $formMapper)
    {
        $formMapper->add('foo', null, array(), array('help' => 'fooHelp'))
        ;
    }
}

and the template that is already created:

    <span class="help-block sonata-ba-field-help">{{ sonata_admin.field_description.help }}</span>
{% endif %}