Service Watchdog manage cron job on package install and deinstall

Currently, if you have some services watched by Service Watchdog, then deinstall the package, the related cron job is left in crontab. But the servicewatchdog_cron.php script is gone.
Then, if you install Service Watchdog again, the defined services to watch are happily waiting there in config.xml from the previous installation, but nothing checks if the cron job is there. So you have to go to the GUI and make some change to force it to check and add the cron job.
This change fixes that - on deinstall the cron job is always removed, and on install the need for the cron job is checked, in case there are already services defined to be watched.