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

examples/sfu-minimal/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"time"
 
 	"github.com/pions/rtcp"
@@ -46,6 +47,11 @@ func main() {
 		panic(err)
 	}
 
+	// Allow us to receive 1 video track
+	if _, err = peerConnection.AddTransceiver(webrtc.RTPCodecTypeVideo); err != nil {
+		panic(err)
+	}
+
 	localTrackChan := make(chan *webrtc.Track)
 	// Set a handler for when a new remote track starts, this just distributes all our packets
 	// to connected peers
@@ -75,7 +81,8 @@ func main() {
 				panic(readErr)
 			}
 
-			if _, err = localTrack.Write(rtpBuf[:i]); err != nil {
+			// ErrClosedPipe means we don't have any subscribers, this is ok if no peers have connected yet
+			if _, err = localTrack.Write(rtpBuf[:i]); err != nil && err != io.ErrClosedPipe {
 				panic(err)
 			}
 		}