Implement Node.cloneNode

Spec:
http://dom.spec.whatwg.org/#dom-node-clonenode

Closes #1240.
auto merge of #1852 : brunoabinader/servo/node-clonenode, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-node-clonenode

This is a sub-task for #1655. Closes #1240.