Rewrite of XML Dump Processing:

Rewrite of XML Dump Processing:
* Full rewrite of the WikiImporter class to a new class, XMLDumpImporter, which uses XMLReader instead of the deprecated PHP SAX interface.
* Fixes for the following bugs:
** XML Dump Processor chokes on unrecognised tags.
** Special:Import does not allow you to import a dump including only log entries.
* Added hooks to allow extensions to handle tags in any part of the XML document.
* Fully source-compatible with the previous interface.