DOM demo improvements
DOM demo improvements

* Remove document.head/body (not implemented well).
* Add Node.parentNode
* Add Node.removeChild
* Fix Node.firstChild/lastChild (was returning native nodes, not pseudo).
* Cache pseudo nodes, so that duplicates aren't created.

NeilFraser/JS-Interpreter,NeilFraser/JS-Interpreter,NeilFraser/JS-Interpreter