Provide default and documentation for APNS_TOPIC

Without a default, PyAPNS2 will raise an error serializing None.

Resolves: #377

Author: matthewh

README.rst

@@ -72,6 +72,7 @@ For WNS, you need both the ``WNS_PACKAGE_SECURITY_KEY`` and the ``WNS_SECRET_KEY
 **APNS settings**
 
 - ``APNS_CERTIFICATE``: Absolute path to your APNS certificate file. Certificates with passphrases are not supported.
+- ``APNS_TOPIC``: The topic of the remote notification, which is typically the bundle ID for your app. If you omit this header and your APNs certificate does not specify multiple topics, the APNs server uses the certificate’s Subject as the default topic.
 - ``APNS_USE_ALTERNATIVE_PORT``: Use port 2197 for APNS, instead of default port 443.
 - ``APNS_USE_SANDBOX``: Use 'api.development.push.apple.com', instead of default host 'api.push.apple.com'.
 

push_notifications/settings.py

@@ -23,6 +23,7 @@
 else:
 	PUSH_NOTIFICATIONS_SETTINGS.setdefault("APNS_USE_SANDBOX", False)
 PUSH_NOTIFICATIONS_SETTINGS.setdefault("APNS_USE_ALTERNATIVE_PORT", False)
+PUSH_NOTIFICATIONS_SETTINGS.setdefault("APNS_TOPIC", "")
 
 # WNS
 PUSH_NOTIFICATIONS_SETTINGS.setdefault("WNS_PACKAGE_SECURITY_ID", None)