Backend Support

Initial backend support, partially taken from django-registration. The
default is already working, need to do something about admin.py,
missing some tests.
Merge branch 'backend'
fixed syntax error when importing
Merge branch 'backend'
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'
fixed migration
Merge branch 'backend'
fixed admin.py that had extra fields
Merge branch 'backend'
fixed sync_plans command bug I introduced
Merge branch 'backend'
Added variables for related model email and name
Merge branch 'backend'
Removing plugin structure
Moved get_related_model to Customer.get_or_create