Job scheduling

I implemented a job scheduler as crontab. I needed to change the interval behavior. Now the interval attribute is not required, because you may use cron.

### Example of configuration
`````
<source>
  ...
  cron            0 9 * * *           # Run at 9 am
  ...
</source>
`````

I used [rufus-scheduler link](https://github.com/jmettraux/) to schedule job.