Class loader is too limited

The new Ascraeus class loader is rather limited when it comes to MODding, it only allows classes to be auto loaded from the phpBB `includes` directory and only when prefixed with `phpbb_`. The class loader should however be able to also load classes from outside this directory (so that mods can include files from outside the phpBB directories) and with an custom class prefix so that MODs aren't forced to use the `phpbb_` prefix.