[SECURITY-167] defend against XXE attacks.

Added a new EntityResolver that will throw an exception if any attempts
are made to load external entities.
Made the transforer use SAX so that we can use out EntityResolover.

As we can't defend against calls that have already parsed the xml (e.g.
DOMSource) if we are parsed one of those throw an exception (which can be
disabled with a System property.