Refactor settings loading.

Get rid of DJANGO_CONF: use default DJANGO_SETTINGS_MODULE variable to choose
which module to load.
Avoid wildcard imports and dont catch ImportError

Catching ImportError and printing "friendlier" message usually just makes more
trouble if it catches ImportError about some other missing module.

Wilcards should be avoided because it makes it hard to notice invalid names.
Continue the use of DJANGO_CONF
Remove non-existant logger from core settings
Merge pull request #13