Add support for importing/exporting files. This can be done by embedding the image...

Add support for importing/exporting files. This can be done by embedding the image as base64 in the XML stream or by copying the images directory manually and pointing the importer to the base images directory.
Currently only backend code available and a few member variables need to be modified to enable the functionality.

Export.php:
* Add <rel> and <sha1base36> elememnts to the XML output
* Add optional <archivename> and <contents> elements to the XML output. <contents> contains an encoding attribute, which is currently only set to base64.
Import.php:
* Add Import::$mImageBasePath which should point to the images/ directory to import from
* Add methods to WikiRevision (terrible name btw) to set the rel, hash, archivename and filesrc.
* Cleanup and made WikiRevision::importUpload working. It's still quite a mess though
OldLocalFiel.php:
* Fix a few timestamp related things from r85635