Implementation of node.parentNode/parentElement
auto merge of #1110 : ILyoan/servo/parentNode, r=jdm

In this implemantation `GetParentNode()` just returns `self.parent_node`.
But according to MDN(https://developer.mozilla.org/en-US/docs/Web/API/Node.parentNode), it says.
"parentNode returns null for the following node types: Attr, Document, DocumentFragment, Entity, and Notation."

should this be checked?

r? @jdm