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/g711_test.go

@@ -0,0 +1,49 @@
+package format
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestG711Attributes(t *testing.T) {
+	format := &G711{}
+	require.Equal(t, "G711", format.String())
+	require.Equal(t, 8000, format.ClockRate())
+	require.Equal(t, uint8(8), format.PayloadType())
+
+	format = &G711{
+		MULaw: true,
+	}
+	require.Equal(t, "G711", format.String())
+	require.Equal(t, 8000, format.ClockRate())
+	require.Equal(t, uint8(0), format.PayloadType())
+}
+
+func TestG711Clone(t *testing.T) {
+	format := &G711{}
+
+	clone := format.Clone()
+	require.NotSame(t, format, clone)
+	require.Equal(t, format, clone)
+}
+
+func TestG711MediaDescription(t *testing.T) {
+	t.Run("pcma", func(t *testing.T) {
+		format := &G711{}
+
+		rtpmap, fmtp := format.Marshal()
+		require.Equal(t, "PCMA/8000", rtpmap)
+		require.Equal(t, "", fmtp)
+	})
+
+	t.Run("pcmu", func(t *testing.T) {
+		format := &G711{
+			MULaw: true,
+		}
+
+		rtpmap, fmtp := format.Marshal()
+		require.Equal(t, "PCMU/8000", rtpmap)
+		require.Equal(t, "", fmtp)
+	})
+}