Let autoloader resolve paths under apps lib directory.

The autoloader only resolved paths to classes where the path was from the appwebroot.

I find it cleaner to have all classes under lib/.

With this patch it can resolve e.g. `OCA\Contacts\AddressBook` to `/var/www/owncloud/apps/contacts/lib/addressbook.php`.

Both variations are still supported.

@bartv2 @VicDeo @icewind1991 @DeepDiver1975