Updated settings file to handle simultaneous PROD and DEV APNS certs

Apple [requires](https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ProvisioningDevelopment.html#//apple_ref/doc/uid/TP40008194-CH104-SW1) developers to create different certificates for their development and production environments.

The app currently supports dev/prod environments by changing the HOSTs in `settings.py` based on `debug=True` or `debug=False`, but there is no mechanism in place for changing the certificate used.

This request is to make room for developers to specify both a prod and dev certificate, and to have the app switch between them in the same way it switches between HOSTs.