* Made it possible to run multiple installer instances in the same cookie domain...

* Made it possible to run multiple installer instances in the same cookie domain without them overwriting or misinterpreting each others' sessions. This is mostly paranoia: one could imagine a situation where the installer from one version of MW becomes insecure when run on the session from another. Did this by making installerData an array indexed by a hash of path and $wgVersion. I could have used $wgCookiePrefix, but that would have broken NoLocalSettings.php, which has no access to AutoLoader and so can't access Installer methods.

* Moved most of the functionality of config/index.php from the file level to a function, so that we can have local variables.