Move supervisor to it's own package

It allows to keep main namespace cleaner

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

supervisor/get_containers.go

@@ -0,0 +1,12 @@
+package supervisor
+
+type GetContainersEvent struct {
+	s *Supervisor
+}
+
+func (h *GetContainersEvent) Handle(e *Event) error {
+	for _, i := range h.s.containers {
+		e.Containers = append(e.Containers, i.container)
+	}
+	return nil
+}