Working

1. For now make class Author inherit from django.contrib.auth.models.User instead of get_user_model, since it does not work properly at model level. Looks like get_user_model does not work with proxy models very well. That seems to be a subtle Django question; not sure how to fix it properly. 

2. Now catching exceptions in zinnia/ping.py when retrieving a list of categories. In tests or before doing syncdb the table zinnia_categories sometimes does not exist yet when this code is executed, so it keeps showing annoying errors.