load models also from engines (+fix filename bug)

Hi,
i'm developing an engine an noticed the app/models/** only picks up app models, not engine ones.
So i fixed that the best way i could. eager_load would have been my preference, but that didn't work.
The table names contain join tables and so the subclasses are the actual models.

Also there was a bug from directory joins missing a /

Thanks for doing the last ones, i had already switched to the gem you released