Implementation fixes around node-insertion and textContent

• Element now responds to tagName
• Node.insertBefore() with a null second argument now appends ("If referenceNode is null, the newNode is inserted at the end of the list of child nodes")
• textContent can be set on Element nodes ("Setting this property on a node removes all of its children and replaces them with a single text node with the given value")
• parentElement/parentNode is now set by insertBefore, not just appendChild