Enable renegotation

Roughly following https://blog.mozilla.org/webrtc/perfect-negotiation-in-webrtc/
but using @negotiationInProcess instead of @peerConnection.signalingState to
make the process a bit more readable/understandable.

Disadvantage: Rollbacks are quite new (half a year in chrome, bit more in Firefox).
But: Races should be rare and all other solutions would require a protocol change
     and would make the process slower.

coffee/remote_peer.coffee

@@ -29,6 +29,7 @@ class palava.RemotePeer extends palava.Peer
     @setupPeerConnection(offers)
     @setupDistributor()
 
+    @offers = offers
     if offers
       @sendOffer()
 
@@ -84,6 +85,10 @@ class palava.RemotePeer extends palava.Peer
       @ready = false
       @emit 'stream_removed'
 
+    @peerConnection.onnegotiationneeded = () =>
+      @negotiationInProcess = true
+      @sendOffer()
+
     @peerConnection.oniceconnectionstatechange = (event) =>
       connectionState = event.target.iceConnectionState
 
@@ -156,12 +161,19 @@ class palava.RemotePeer extends palava.Peer
       @peerConnection.addIceCandidate(candidate)
 
     @distributor.on 'offer', (msg) =>
+      if @negotiationInProcess
+        # we're in a race (both peers sent an offer at the same time)
+        # case 1: we have offer precedence, thus we throw away the other offer
+        return if @offers
+        # case 2: the peer has offer precende, thus we back off and roll back
+        @peerConnection.setLocalDescription({type: "rollback"})
       @peerConnection.setRemoteDescription(new RTCSessionDescription(msg.sdp))
       @emit 'offer' # ignored so far
       @sendAnswer()
 
     @distributor.on 'answer', (msg) =>
       @peerConnection.setRemoteDescription(new RTCSessionDescription(msg.sdp))
+      @negotiationInProcess = false
       @emit 'answer' # ignored so far
 
     @distributor.on 'peer_updated_status', (msg) =>