don't use get_user_model() in models.py
don't use get_user_model() in models.py

fixes a problem with Django 1.7 and the new AppConfig system

arneb/django-messages,tobiasgoecke/django-messages,andyzsf/django-messages,nikhil-above/django-messages,Chris7/django-messages,gustavoam/django-messages,JordanReiter/django-messages,JordanReiter/django-messages,fpischedda/django-messages,cca/django-messages,arneb/django-messages,brajeshvit/Messagemodule,procrasti/django-messages,procrasti/django-messages,fpischedda/django-messages,brajeshvit/Messagemodule,gustavoam/django-messages,tobiasgoecke/django-messages,Chris7/django-messages,andyzsf/django-messages,nikhil-above/django-messages