Implement Node::replaceChild()

Implements Node:replaceChild() according to spec below:
http://dom.spec.whatwg.org/#concept-node-replace

Closes #1430.
auto merge of #1504 : brunoabinader/servo/node-replace-child, r=Ms2ger

Implements Node:replaceChild() according to spec below:
http://dom.spec.whatwg.org/#concept-node-replace

Closes #1430.