Move django_cron/__init__.py code to helper.py and core.py respectively?

[chachra]

For Django 1.9 we've had to import the models inside the functions rather than at the top (globally). Seems a bit of a quick solution and most of the code there easily fits in the helper.py (functions etc.) and core.py (the core classes we use).

Thoughts @dfryta @mjaworski @name-no @mateusz-sikora ? See #70