Update offsite_postfinance.py

Added some comments to speed up setup of PostFinance payment backend. Data for IPN is not stored on order, it should be pulled from the REQUEST data. Typos
Update utils.py

Cleaned up, removed the settings.POSTFINANCE_SECRET_KEY to allow to pass a SHA-OUT key
Merge branch 'master' of https://github.com/gabejackson/django-shop-postfinance