Switched from user to related_model

Reworked backend to not use code in __init__.py, refactored to use
related_model,
Renamed search fields in admin.py
Moved user specific code from admin to backend
Removed commented test, updated settings docs
Renamed CUSTOMER_RELATED_MODEL

Renamed CUSTOMER_RELATED_MODEL to DJSTRIPE_CUSTOMER_RELATED_MODEL
Merge branch 'backend'