add initial tutor model

oser_cs/oser_cs/settings/default.py

oser_cs/users/admin.py

oser_cs/users/models.py

oser_cs/users/tests/__init__.py

oser_cs/users/tests/test_custom_user.py

oser_cs/users/tests/test_utils.py

oser_cs/users/utils.py