Get rid of jdom dependency in xwiki-xml module

Currently jdom is only used as a workaround for the problem with w3c dom where it is not possible to set the DocType of the document once it is serialized. But this can be overcome using a mechanism like what is mentioned in http://www.ibm.com/developerworks/library/x-tipmvdom.html. The only difference would be that we must use adoptNode() mode method instead of importNode() method to avoid the extra overhead of copying nodes.