convert Tracks into Medias and Formats (#155)

* 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

pkg/format/vorbis_test.go

@@ -0,0 +1,45 @@
+package format
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestVorbisAttributes(t *testing.T) {
+	format := &Vorbis{
+		PayloadTyp:    96,
+		SampleRate:    48000,
+		ChannelCount:  2,
+		Configuration: []byte{0x01, 0x02, 0x03, 0x04},
+	}
+	require.Equal(t, "Vorbis", format.String())
+	require.Equal(t, 48000, format.ClockRate())
+	require.Equal(t, uint8(96), format.PayloadType())
+}
+
+func TestTracVorbisClone(t *testing.T) {
+	format := &Vorbis{
+		PayloadTyp:    96,
+		SampleRate:    48000,
+		ChannelCount:  2,
+		Configuration: []byte{0x01, 0x02, 0x03, 0x04},
+	}
+
+	clone := format.Clone()
+	require.NotSame(t, format, clone)
+	require.Equal(t, format, clone)
+}
+
+func TestVorbisMediaDescription(t *testing.T) {
+	format := &Vorbis{
+		PayloadTyp:    96,
+		SampleRate:    48000,
+		ChannelCount:  2,
+		Configuration: []byte{0x01, 0x02, 0x03, 0x04},
+	}
+
+	rtpmap, fmtp := format.Marshal()
+	require.Equal(t, "VORBIS/48000/2", rtpmap)
+	require.Equal(t, "configuration=AQIDBA==", fmtp)
+}