feat(iframe_api): get number of participants

doc/api.md

 api.removeEventListeners(["incomingMessage", "outgoingMessageListener"]);
 ```
 
+You can get the number of participants in the conference with the following code:
+```
+var numberOfParticipants = api.getNumberOfParticipant();
+```
+
 You can remove the embedded Jitsi Meet Conference with the following code:
 ```
 api.dispose()

modules/API/external/external_api.js

     });
 }
 
+/**
+ * Adds given number to the numberOfParticipants property of given APIInstance.
+ * @param {JitsiMeetExternalAPI} APIInstance the instance of the
+ * JitsiMeetExternalAPI
+ * @param {int} number - the number of participants to be added to
+ * numberOfParticipants property (this parameter can be negative number if the
+ * numberOfParticipants should be decreased).
+ */
+function changeParticipantNumber(APIInstance, number) {
+    APIInstance.numberOfParticipants += number;
+}
+
 /**
  * Constructs new API instance. Creates iframe element that loads
  * Jitsi Meet.
 
     this.eventHandlers = {};
 
+    this.numberOfParticipants = 1;
+    this._setupListeners();
+
     id++;
 }
 
         this.removeEventListener(events[i]);
 };
 
+/**
+ * Returns the number of participants in the conference.
+ * NOTE: the local participant is included.
+ * @returns {int} the number of participants in the conference.
+ */
+JitsiMeetExternalAPI.prototype.getNumberOfParticipant = function() {
+    return this.numberOfParticipants;
+};
+
+/**
+ * Setups listeners that are used internally for JitsiMeetExternalAPI.
+ */
+JitsiMeetExternalAPI.prototype._setupListeners = function() {
+    this.postis.listen("participant-joined",
+        changeParticipantNumber.bind(null, this, 1));
+    this.postis.listen("participant-left",
+        changeParticipantNumber.bind(null, this, -1));
+};
+
 /**
  * Removes the listeners and removes the Jitsi Meet frame.
  */