Move internal event.Event model to pkg namespace

[UiP9AV6Y]

Golang applications which want to create integration for the icinga-notifications event processing API (/process-event) currently need to maintain their own Event model implementation as the one this project provides is hidden in the internal/event namespace. ideally event.Event and event.Severity live in the pkg namespace (e.g. pkg/model) and the internal event.Event implementation can be rewritten to something like this

type Event struct {
	model.Event

	Time     time.Time
	SourceId int64
	ID       int64
}

this would follow to approach taken for the plugin system, where the models are public too (pkg/plugin)