File upload plugins / video transcoding / security fixes / unit tests.

 * Add plugins for XML validation and File extension and mime-type checking.
 * Remove code duplication between modules/xerte/engine/{save|upload}.php and modules/site/engine/{save|upload.php}
 * Add transcoding script to convert flvs to mp4s assuming appropriate binaries are present. (see cron/transcoding.php)
 * Some code cleanups (reformatting)
 * Some security improvements (addition of more .htaccess files to stop people browsing LOs on Apache server, try and stop PHP files from being executed within the USER-FILES tree etc).
 * Remove undefined variable issues from /drawing.php
 * Add some unit tests (!!!!!!) (See tests/).