! PM notifications were shown in the wrong order. I'm not sure why nobody ever reported that. Well, I reported it to myself. I'm a good tester. If a little slow. (PersonalMessage.php)
+ The install script no longer needs to be moved to the forum's root. Wedge will automatically run it from its install/ folder if it detects that the Settings.php file is missing (which has always been 'the' way to force a Wedge reinstall anyway.)

+ The install script can no longer be run by someone else than the admin. (index.php, install.php)

+ 'Empty' folders such as media, attachments, gz and plugins are now automatically created by Wedge at install time. This allows me to avoid enforcing write access to them. Making life easier for everyone. Plus, it's faster to upload because there are less round trips. (index.php, OriginalFiles.php, install.php)

@ Note: assets/avatars doesn't seem to need to be writable. I think some very old versions might have used the avatar folders to store uploaded avatars, when they're now either in the attachments folder, or in a custom folder that we know nothing about at install time. So, I took the liberty to remove it from the list. So, now, only the assets/smileys folder needs to be writable, and it's only if you want to upload new smiley sets by HTTP... Which is currently broken.

@ Known issue: install files won't delete themselves after install, you'll have to remove the folder manually. I'll fix that later. Generally speaking, this new installer needs some testing. I successfully installed a test forum with it, but I'd never say it's 100% safe because of that. If you have problems with installing, get the previous commit from GitHub and you should be fine.