splitting the u_short into 8-bit fields.

easy to extract pt
added detection for STUN, DTLS, RTP, RTCP. Only writing RTP packets.

based on:
    From http://tools.ietf.org/html/rfc5764#section-5.1.2
    http://tools.ietf.org/html/rfc3550#appendix-A.1
    http://tools.ietf.org/html/rfc3550#section-12
    http://www.iana.org/assignments/rtp-parameters/rtp-parameters.xml

This works very well for WebRTC which multiplexes everything on a
single port.

TODO: writing RTCP packets