Migrate SDP generation to Unified Plan

This commit has breaking changes. This API change means we
can no longer support an arbitrary number of receivers. For every track
you want to receive you MUST call PeerConnection.AddTransceiver

We do now support sending an multiple audio/video feeds. You can see
this behavior via gstreamer-receive and gstreamer-send currently.

Resolves #54

track.go

@@ -11,7 +11,11 @@ import (
 	"github.com/pions/webrtc/pkg/media"
 )
 
-const rtpOutboundMTU = 1400
+const (
+	rtpOutboundMTU          = 1400
+	trackDefaultIDLength    = 16
+	trackDefaultLabelLength = 16
+)
 
 // Track represents a single media track
 type Track struct {