Implement DOMImplementation::createDocumentType

Implementation according to spec:
http://dom.spec.whatwg.org/#dom-domimplementation-createdocumenttype

Closes #1489.
auto merge of #1497 : brunoabinader/servo/domimpl-create-document-type, r=Ms2ger

Implementation according to spec:
http://dom.spec.whatwg.org/#dom-domimplementation-createdocumenttype

Closes #1489.