add updating domains' ip periodly feature

when we added trusted domains , for many popular domains, for example facebook, google, qq etc, its domains' ip will change frequently, so I add this feature to get more domains' ip. If u want to use this feature, add UpdateDomainInterval to wifidog.conf, for instance:

UpdateDomainInterval 60

which means update every 1 hour, if our checkinterval is 60 seconds.