update to FlashMessenger view helper to allow for classes on separator

This Update to FlashMessenger allows for the user to utilise the classes within the separator as well as the opening part for the render method;

```php
$flashMessenger = $this->flashMessenger();

$flashMessenger
    ->setMessageOpenFormat('<div%s>')
    ->setMessageSeparatorString('</div><div%s>')
    ->setMessageCloseString('</div>');

echo $flashMessenger->render('error', array('alert', 'alert-error'));
```