Support for apps in different folders

I have all my apps under apps/ folder. If I try to register MongoAdmin on a model from the app that has the following structure: project.apps.app.models it won't work.
This change should fix this, and "app_label" should become the whole path to the app.
I think all of the `.` in the regular expressions should be changed to `\.`.  Otherwise this allows any character in the url.