Implement Node.isEqualNode

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

Closes #1645.
auto merge of #1646 : brunoabinader/servo/node-isequalnode, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-node-isequalnode

This is a sub-task for #1655. Closes #1645.