Add integration for PrivateMSG module

- Add integration for PrivateMSG module: Messages sent through the PrivateMSG module will be delivered via a push notification to all recipients with a valid device token. Integration requires PrivateMSG module 7.x-1.x and needs to be activated on the Push Notifications configuration page: admin/config/services/push_notifications/configure
- Small bugfix: Remove dpm statement in Google Cloud Messaging function