Allow to access easily app settings without prefix.

It is only a feature proposal.

https://github.com/jezdez/django-appconf has a nice property that application settings can be accessed without all that ``MYAPPLICATIONLONGNAMEPREFIX_`` stuff (while project settings overrides still need to be prefixed, of course). But ``django-appconf`` (as many other app setting handlers, **excluding yours**) clobber global settings - that's unacceptable.

This pull request tries to seaminglessly add such a possibility.

Reasonable:

When smb do
```python
from myapp.conf import settings
```
it is obvious that in most cases he wants to access application settings, so repeating MYAPP_ at every access is boring.