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.
When inside a set, a '.' is a '.'.  It doesn't hurt to escape it, but it's not necessary. 

```
>>> import re
>>> re.match("[.]", ".")
<_sre.SRE_Match object at 0x106ceca58>
>>> re.match("[.]", "s")
>>> re.match("[\.]", "s")
>>> re.match("[\.]", ".")
<_sre.SRE_Match object at 0x106cec578>
```