Refs #41, adds a tentative database schema for notifications.
Refs #41, adds a Notification model.
Refs #41, adds get-notifications and set-notifications-read.

Also makes some changes/fixes to the Notification model.
Refs #41, removes message objects from notifications.
Refs #41, implements an updated schema for notifications.

Things that still have to be done:
* Provide some text with the notification so the client can tell the user what it is.
* Add/update notifications on events.
Refs #41, adds a Notification::add() function and updates the config parameters.

The Notification::add() function should work with every type of notification, it just needs to be fed parameters depending on the type. This will be implemented as Notification::add_for_follow() etc.
Refs #41, adds notifications for following, upvoting, remixing, and mentioning.
Refs #41, adds a database trigger to delete notifications with no notification_objects children.
Refs #41, deletes notifications on unfollow, delete-upvote, etc.

I have removed the trigger from the database that deletes a notification if it doesn't have any notification_object children. Instead, Notification::get_latest() just doesn't return notifications with no children. I have removed the trigger because MySQL doesn't support calling delete triggers when that deletion is cascaded by foreign keys.
Merge pull request #46 from RyffProject/feature/notifications

Feature/notifications, see #41