Update the User model references for Django 1.5+
Update the User model references for Django 1.5+

Because custom user models are allowed in Django 1.5 and above,
it is recommended to use setttings.AUTH_USER_MODEL.

Failing to do so will break installations with a custom model.

claudep/django-calendarium,bitmazk/django-calendarium,claudep/django-calendarium,claudep/django-calendarium,bitmazk/django-calendarium,claudep/django-calendarium,bitmazk/django-calendarium