Option to power off the backend

Since the server already offers the possibility to wake up backends using WOL, I missed the possibility to put a backend back to sleep again. So I decided to implement the functionality to shut down, suspend, hibernate or reboot the backend. By default, this can only be done from an administrator's account, but you can also make it available to users via the settings.

If you like, you can pull the changes.
If you added a method like `Yii::app()->powerOffManager->getAllowedActions()` which would return e.g. `array('poweroff','suspend')` you could use that here as well as for the poweroff modal cache variation.