* begin to add a rtp mixer. For now it support mpeg audio and ac3(untested).

 TODO: (a lot of work)
    - gather multiple small frames in one packets (not really important).
    - add mpeg 1/2 video
    - export a sdp file (needed for all next codecs).
    - add mpeg4 audio/video
    - add ts/ps over RTP.
    - perhaps ogg over RTP if something is abled to read that.

 command: #rtp{dst=ip,port=port,[mux=muxer]} (mux not yet implemented)

 It automaticaly uses ip:port, ip:port+2, ip:port+4 ... (port should
 be odd btw)