Implement DOMImplementation::createDocument

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

Closes #1509.
Added content tests for DOMImplementation.createDocument
auto merge of #2072 : brunoabinader/servo/domimpl-createdocument, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-domimplementation-createdocument

Closes #1509.