add dependencies between services(server, website, and so on)

[moonrailgun]

Hi. Was wondering if Tianji supports dependancies? Like I'd like to be able to say these websites are running on server X. If the server is down, then don't send me an alert for every site. Or some other service depends on the internet, so if my router is down, then don't bother alerting me about every single thing that needs internet..

[moonrailgun]