Add null-agent

For monitoring hosts where agent installation isn't an option (such as external ones), it would be nice to allow the registration of a null-agent, meaning nothing except for the IP address. With this,it'd be possible to allow for checking web scenarios, ping checks or simple checks and linking them to a hostname. I find this a better way of doing such tests than linking them to another host such as the zabbix server itself. I would guess adding this to zabbix hopefully isn't a very time-consuming job.