Use a cron instead of threads to update printers status

The implementation with threads was blocking the loading of the
server in multiprocess.  Using a cron will lower the frequency of
the updates but at least it is simple and reliable.

Fixes #14
Do not write the printer status if it has not changed

Avoid unnecessary UPDATE every minute
Merge pull request #15 from guewen/base_report_to_printer-cron-issue14

Use a cron instead of threads to update printers status