basic view to handle notifications

The view just kicks off a celery task to handle the notification, then returns a blank HTTP 200 response.
The celery task currently only confirms subscriptions, or updates summary data. The task goes through each message in the notification and builds a date range based on goal updates, so we can use just one HTTP request to get our summary data. It could probably use more tests.

TODO:
- [ ] Save goal updates to the DB https://github.com/orcasgit/django-misfit/compare/notification?expand=1#diff-1c4501bba1559aa5f549dce41d998ebeR54
- [ ] Save profile, device, session, and sleep updates to the DB https://github.com/orcasgit/django-misfit/compare/notification?expand=1#diff-1c4501bba1559aa5f549dce41d998ebeR64