Add RTP/MJPEG decoder and encoder (#165)

* rename format.JPEG into format.MJPEG

* add examples/client-publish-format-mjpeg

* add rtp/mjpeg decoder and encoder

examples/client-read-format-mpeg4audio/main.go

@@ -57,6 +57,7 @@ func main() {
 		// decode MPEG4-audio AUs from the RTP packet
 		aus, _, err := rtpDec.Decode(pkt)
 		if err != nil {
+			log.Printf("ERR: %v", err)
 			return
 		}