img tag has 'empty' content model as per html5 spec

As per the HTML4 spec (http://www.w3.org/TR/html401/struct/objects.html#h-13.2) and HTML5 spec (http://www.w3.org/TR/html5/embedded-content-0.html#the-img-element), the IMG tag will never have children. 

This patch disallows creation of IMG-tags with child elements.