Remove code duplication in track/transceiver code

Have AddTrack call transceiver code. Also update AddTransceiverFromKind
to call AddTransceiverFromTrack

Relates to #207

rtptransceiver.go

@@ -48,5 +48,7 @@ func (t *RTPTransceiver) Stop() error {
 			return err
 		}
 	}
+
+	t.Direction = RTPTransceiverDirectionInactive
 	return nil
 }