Go through all active notifications (active means they are not notified
again) and check if their state has changed. If so, mark as inactive and
allow for re-notification of the same type and data.
* Added main mail class
* notification class extends mail class
* Added mail template for IDLE workers
* Added notification table to SQL structure
This works already but once notified the status is not reset as soon as
a worker is active again. Need to think of a system to do that
automatically.
Addresses #116