Guard against multiple SipProviders

This change adds ID attribute to audio element when attaching to document, checks against existence of one such element during component mounting and properly removes audio element when SipProvider is unmounted.

src/components/SipProvider/index.js

@@ -111,9 +111,14 @@ export default class SipProvider extends React.Component {
   }
 
   componentDidMount() {
-    // TODO check against having two instances of SipProvider in one app, which is not allowed
-
+    if (window.document.getElementById('sip-provider-audio')) {
+      throw new Error(`Creating two SipProviders in one application is forbidden. If that's not the case `
+                     +`then check if you're using "sip-provider-audio" as id attribute for any existing `
+                     +`element`);
+    }
+
     this.remoteAudio = window.document.createElement('audio');
+    this.remoteAudio.id = 'sip-provider-audio';
     window.document.body.appendChild(this.remoteAudio);
 
     this.reconfigureDebug();
@@ -137,6 +142,7 @@ export default class SipProvider extends React.Component {
   }
 
   componentWillUnmount() {
+    this.remoteAudio.parentNode.removeChild(this.remoteAudio);
     delete this.remoteAudio;
     if (this.ua) {
       this.ua.stop();