Patch #407965: Improve Level 2 conformance of minidom

- addition of a DocumentFragment implementation and createDocumentFragment method
- proper setting of ownerDocument for all nodes
- setting of namespaceURI to None in Element as a class attribute
- addition of setAttributeNodeNS and removeAttributeNodeNS as aliases
  for setAttributeNode and removeAttributeNode
- support for inheriting from DOMImplementation to extend it with
  additional features (to override the Document class)
in pulldom:
- support for nodes (comment and PI) that occur before he document element;
  that became necessary as pulldom now delays creation of the document
  until it has the document element.