Make Document a Node.
auto merge of #1079 : jdm/servo/docnode, r=metajack

The bit I don't like about these changes is that I ended up hiding the document node from the CSS selecting/matching code, so it continues thinking of the document's first child as the root. When I tried to send the full tree including the document node to layout, the layout code refused to create any child flows. When I sent the document's first child without hiding the document, it saw inherited values for properties like font-family, and later tried to treat the document node as an Element when searching for named nodes.