add start and end time, add meeting, add permission file, update api tests and docs

oser_backend/tests/test_visits/test_attached_file.py

oser_backend/tests/test_visits/test_visit.py

oser_backend/tests/test_visits/test_visit_api.py

oser_backend/visits/admin.py

oser_backend/visits/factory.py

oser_backend/visits/migrations/0016_auto_20180522_2139.py

oser_backend/visits/migrations/0017_auto_20180522_2233.py

oser_backend/visits/models.py

oser_backend/visits/serializers.py

oser_backend/visits/views.py