python 2.7

[davidbarkhuizen]

• doc note on criterion for matching saml user identity to django user (namely, on username)
• only import jwt_encode from rest_auth.utils if USE_JWT setting is True
• user authentication backend now configurable via AUTHENTICATION_BACKEND setting, defaulting to django.contrib.auth.backends.ModelBackend'
• added login callback method configurable via TRIGGER.LOGIN setting
• some info logging