Add rtcp.RawPacket type

This will be returned by Unmarhsal when an unknown type is seen
in the packet stream, avoiding an error and allowing the user to
write their own packet handling code for that type if desired.

Relates to #119

pkg/rtcp/packet.go

@@ -36,10 +36,9 @@ func Unmarshal(rawPacket []byte) (Packet, Header, error) {
 		p = new(PictureLossIndication)
 
 	default:
-		return nil, h, errWrongType
+		p = new(RawPacket)
 	}
 
 	err = p.Unmarshal(rawPacket)
 	return p, h, err
-
 }