Add support for notification parsing

- Add MisfitNotification class
- Verify SNS signatures
- Confirm subscriptions
- Deserialize the messages to a list of dicts (and arrow datetimes)

The key/cert pair are for testing so we can sign fake Amazon SNS POSTs. I have tested with real POSTs as well. This cryptography library is very useful. Hopefully https://github.com/pyca/cryptography/pull/1499 makes it into a release soon. I've pinned to a specific commit in the meantime so we can be reasonably certain it will run smoothly.