Initialize autoloader as the very first thing.

Libraries and the login panel may need autoloader's
PSR-0 classes too.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5443 2fea6a4d-a838-0410-917b-93a53c48e9d2
Merge remote-tracking branch 'upstream/master' into jquery-ui-integration