Revert "Make Document a Node." for breaking Acid1

This reverts commit 4e47d59165d186d0938fe9ffd726b2c1b83d50f4.

Conflicts:

	src/components/script/dom/domparser.rs
auto merge of #1128 : pcwalton/servo/revert-document-node, r=metajack

This reverts commit 4e47d59165d186d0938fe9ffd726b2c1b83d50f4.

Conflicts:

	src/components/script/dom/domparser.rs

r? @metajack