Handle PSR0 allowing dirs to mean either or both underscore/namespace

PSR0 wants to be quantum, so they let a directory separator mean
a namespace or an underscore at the same time and you can only
tell when you try to observe it...

Or rather, now you can namespace your model files if you need to
do so to spec them with PHPSpec (which doesn't support underscores)
or if you feel like it for some other reason.