Support running annotations and relationships through the notifications system

This is something I came up with when looking into upgrading a couple of plugins for 1.9.

The patch allows content owners to receive notifications through the notifications system instead of using notify_user() on content creation.

However the main point is to allow subscription and notification plugins to monitor site activity and send notifications to followers also when an action creates a relationship or an annotation. (Currently master supports only entities).

Use cases:
 1. User A receives a notification when user B joins the group C.
 2. User A receives a notification when user B likes a post by user C.
 3. Users A, B and C like a post by user D. Each of these events are run through the notifications system. Instead of receiving three separate (site)notifications the first one is updated to "Users A, B and C like your post".