Implement SFU example

`sfu` example shows how to build a simple broadcast only SFU

Resolves #63

examples/sfu/jsfiddle/demo.js

@@ -0,0 +1,62 @@
+/* eslint-env browser */
+var log = msg => {
+  document.getElementById('logs').innerHTML += msg + '<br>'
+}
+
+window.createSession = isPublisher => {
+  let pc = new RTCPeerConnection({
+    iceServers: [
+      {
+        urls: 'stun:stun.l.google.com:19302'
+      }
+    ]
+  })
+  pc.oniceconnectionstatechange = e => log(pc.iceConnectionState)
+  pc.onicecandidate = event => {
+    if (event.candidate === null) {
+      document.getElementById('localSessionDescription').value = btoa(pc.localDescription.sdp)
+    }
+  }
+
+  if (isPublisher) {
+    navigator.mediaDevices.getUserMedia({video: true, audio: false})
+      .then(stream => pc.addStream(document.getElementById('video1').srcObject = stream))
+      .catch(log)
+    pc.onnegotiationneeded = e => {
+      pc.createOffer()
+        .then(d => pc.setLocalDescription(d))
+        .catch(log)
+    }
+  } else {
+    pc.createOffer({offerToReceiveVideo: true})
+      .then(d => pc.setLocalDescription(d))
+      .catch(log)
+
+    pc.ontrack = function (event) {
+      var el = document.getElementById('video1')
+      el.srcObject = event.streams[0]
+      el.autoplay = true
+      el.controls = true
+    }
+  }
+
+  window.startSession = () => {
+    let sd = document.getElementById('remoteSessionDescription').value
+    if (sd === '') {
+      return alert('Session Description must not be empty')
+    }
+
+    try {
+      pc.setRemoteDescription(new RTCSessionDescription({type: 'answer', sdp: atob(sd)}))
+    } catch (e) {
+      alert(e)
+    }
+  }
+
+  let btns = document.getElementsByClassName('createSessionButton')
+  for (let i = 0; i < btns.length; i++) {
+    btns[i].style = 'display: none'
+  }
+
+  document.getElementById('signalingContainer').style = 'display: block'
+}