Add untagged container

The PR is a try to fix #3

In the original library there was no way to render the following:
```html
<!-- The image -->
<img src="home.jpg" />
<!-- End of the image -->
```

With this PR one can use the following:
```php
$container = html('container');

$container->addChild(html('comment', 'The image'));
$container->addChild(html('img', array('src' => 'home.jpg')));
$container->addChild(html('comment', 'End of the image'));

echo $container;
```