Add ability to signal container not just processes

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

update.go

@@ -25,5 +25,16 @@ func (h *UpdateEvent) Handle(e *Event) error {
 			return ErrUnknownContainerStatus
 		}
 	}
+	if e.Signal != nil {
+		// signal the pid1/main process of the container
+		processes, err := container.Processes()
+		if err != nil {
+			return err
+		}
+		if len(processes) == 0 {
+			return ErrProcessNotFound
+		}
+		return processes[0].Signal(e.Signal)
+	}
 	return nil
 }