#7 support parent-child relation between monitors

[nicolas de loof]

Some monitors may handle data in a parent-child way :

A web monitor will track request response time, and the web-http.200 / web-http.500 will track same request but distinct success and failure. The two later can be considered child monitors of the first one.

This parent/child could the be used in a report / or the web-ui to group the monitors in a master/detail representation.