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

mediaengine.go

@@ -4,6 +4,7 @@ package webrtc
 
 import (
 	"strconv"
+	"strings"
 
 	"github.com/pions/rtp"
 	"github.com/pions/rtp/codecs"
@@ -164,6 +165,18 @@ func (t RTPCodecType) String() string {
 	}
 }
 
+// NewRTPCodecType creates a RTPCodecType from a string
+func NewRTPCodecType(r string) RTPCodecType {
+	switch {
+	case strings.EqualFold(r, "audio"):
+		return RTPCodecTypeAudio
+	case strings.EqualFold(r, "video"):
+		return RTPCodecTypeVideo
+	default:
+		return RTPCodecType(0)
+	}
+}
+
 // RTPCodec represents a codec supported by the PeerConnection
 type RTPCodec struct {
 	RTPCodecCapability