add systemd timer units replacing cronjobs

Systemd timer units, while preserving functionality of cronjobs,
provide some additional features:
- dependency on cron can be dropped
- timer units next expiration is clearly visible in "systemctl list-timers"
- unit customization rules provide cleaner way for administrator to modify
  default parameters, enable, disable and override individual timers
Merge branch 'master' of https://github.com/zdzichu/sysstat

Merge new features added by Tomasz Torcz.