Issue #1658132 by jca, SerkanB, haagendazs: Add Rules integration.

Provides a rules action (Send a push notification) that sends out a push notification to users with registered device tokens as well as a number of rules events to allow other modules to react to the following events:
- After deleting a device token (push_notifications_after_token_delete)
- After saving a new device token (push_notifications_after_token_insert)
- After sending an Android push notification (C2DM) (push_notifications_after_c2dm_send)
- After sending an Android push notification (GCM) (push_notifications_after_gcm_send)
- After sending an iOS push notification (APNS) (push_notifications_after_apns_send)
- Before deleting a device token (push_notifications_before_token_delete)
- Before saving a new device token (push_notifications_before_token_insert)
- iOS feedback service was completed (push_notifications_after_apns_feedback)