Some experimental support for including upload data in XML export, as specced. Usable...

Some experimental support for including upload data in XML export, as specced. Usable with --uploads option on dumpBackups.php
Some issues:
* doesn't respond to the current v full-history modes
* won't pick up file entries with no corresponding page
* on default config, you may end up with 'localhost' in URLs :P
* synchronization and mutability problems
** DB lookups are made in the middle of the dump and may not be in sync with state at the beginning of the dump.
** URL to current version at time of dump could point to a different, future version in future