Add 'type' property to all 'signal' messages

Only some signaling message have 'type' properties. The others rely on the key name that is in use.

Having a 'type' in all messages improves console debug logs (since I can include the type in the output).

index.js

@@ -254,6 +254,7 @@ class Peer extends stream.Duplex {
       }
     } else {
       this.emit('signal', { // request initiator to renegotiate
+        type: 'transceiverRequest',
         transceiverRequest: { kind, init }
       })
     }
@@ -388,6 +389,7 @@ class Peer extends stream.Duplex {
       } else {
         this._debug('requesting negotiation from initiator')
         this.emit('signal', { // request initiator to renegotiate
+          type: 'renegotiate',
           renegotiate: true
         })
       }
@@ -908,6 +910,7 @@ class Peer extends stream.Duplex {
     if (this.destroyed) return
     if (event.candidate && this.trickle) {
       this.emit('signal', {
+        type: 'candidate',
         candidate: {
           candidate: event.candidate.candidate,
           sdpMLineIndex: event.candidate.sdpMLineIndex,