Implement TElement for JSRef<Element>
Implement TNode for JS<Node>
Added ParentNode.querySelector skeleton
Implement querySelector for Document
Implement querySelector for DocumentFragment
Implement querySelector for Element
Added content tests for querySelector
Fixed traversal issues
auto merge of #2591 : brunoabinader/servo/document-queryselector-v3, r=jdm

This is a subtask for #2254 & #2576.

Spec:
http://dom.spec.whatwg.org/#dom-parentnode-queryselector