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/jpeg.go

@@ -0,0 +1,42 @@
+package format
+
+import (
+	"github.com/pion/rtp"
+)
+
+// JPEG is a JPEG format.
+type JPEG struct{}
+
+// String implements Format.
+func (t *JPEG) String() string {
+	return "JPEG"
+}
+
+// ClockRate implements Format.
+func (t *JPEG) ClockRate() int {
+	return 90000
+}
+
+// PayloadType implements Format.
+func (t *JPEG) PayloadType() uint8 {
+	return 26
+}
+
+func (t *JPEG) unmarshal(payloadType uint8, clock string, codec string, rtpmap string, fmtp string) error {
+	return nil
+}
+
+// Marshal implements Format.
+func (t *JPEG) Marshal() (string, string) {
+	return "JPEG/90000", ""
+}
+
+// Clone implements Format.
+func (t *JPEG) Clone() Format {
+	return &JPEG{}
+}
+
+// PTSEqualsDTS implements Format.
+func (t *JPEG) PTSEqualsDTS(*rtp.Packet) bool {
+	return true
+}