add a cron - hdd cache

[timse201]

due to technical limitations the hdd cache can not expire
these days some small hoster have a webinterface to add crons, too

so adding a cron inside the plugin would be helpful
if the user wants to use this option there should be a small notification that wp-cron.php should get hitted by a server cron or a service provider

when selecting 0 the cron should be disabled

until that the option should be disabled and there should be an information while switching to hdd (#62)

#32