Initial Django 1.7 compatibility fix

When rosetta tries to load every INSTALLED_APP, it fetches app modules. However, in Django 1.7, importing some INSTALLED_APPS can fetch AppConfig instances instead of modules, and they have no `__file__` attribute. You then need to find the app module for the AppConfig instance.
This allows rosetta to properly seek locale folders.
Update poutil.py
Merge pull request #105 from artscoop/patch-1

Initial Django 1.7 support