* split tracks from medias
* move tracks into dedicated package
* move media into dedicated package
* edit Medias.Marshal() in order to return SDP
* add medias.Find() and simplify examples
* improve coverage
* fix rebase errors
* replace TrackIDs with MediaIDs
* implement media-specific and track-specific callbacks for reading RTCP and RTP packets
* rename publish into record, read into play
* add v2 tag
* rename tracks into formats
