Implement DOMImplementation::createHTMLDocument

Spec:
http://dom.spec.whatwg.org/#dom-domimplementation-createhtmldocument

Closes #1510.
auto merge of #1523 : brunoabinader/servo/domimpl-createhtmldocument, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-domimplementation-createhtmldocument

Closes #1510.