Don't require passing a root element to Document::new (needed for issue #888).
Address review comments.
Remove unnecessary mutability.
Remove Document::set_root.

All callers have an AbstractDocument, and this makes my life easier later.
Cleanup some code.
auto merge of #1023 : Ms2ger/servo/cleanup, r=jdm
auto merge of #1016 : Ms2ger/servo/ownerdoc, r=jdm