Add worker montioring to template and worker class

* Add configurable monitoring for workers into template
* Store monitoring status in DB

First commit for #116
Progress on notifications

* 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