Notification system

DiscoverD should allow a highly customizable notification system. By that, I mean a system of plugins, where a plugin is built to interact with another service like a SMTP server (postfix is preferred) or a dialer. Some important changes in the architecture have to be done: each service detected by the collector must have a "monitoring status" (type: boolean), a notification being sent only if the service can be considered as monitored. Moreover, the whole system must implement options allowing a given number of checks before considering the service status as changed.