Issue 144 : Add notification handler and settings

Using notifications on live pool, needs some more testing but worked fine on a testnet.

Merging into next. Please **upgrade your database** using the supplied SQL file (`sql/issue_144_notification_upgrade.sql`) to add the new tables. If you have the notifications table already you can drop it and re-create it using this SQL file. Previous notifications will be lost, but that will not affect the system.

**Changes**

* New table: `notifications`
 * Holds all notifications sent by the system
* New table: `notification_settings`
 * Per-user notificiation settings
 * All notifications **disabled** by default!
* New cron: `notifications.php`
 * Sends out notifications for idle workers

Fixes #144