Adds a option to set custom callstats script.

doc/API.md

     10. disableSimulcast - boolean property. Enables/disables simulcast.
     11. enableWindowOnErrorHandler - boolean property (default false). Enables/disables attaching global onerror handler (window.onerror).
     12. disableThirdPartyRequests - if true - callstats will be disabled and the callstats API won't be included.
-    13. analyticsScriptUrl - (optional) custom url to search for the analytics lib, if missing js file will be expected to be next to the library file (the location it is sourced from) 
+    13. analyticsScriptUrl - (optional) custom url to search for the analytics lib, if missing js file will be expected to be next to the library file (the location it is sourced from)
+    14. callStatsCustomScriptUrl - (optional) custom url to access callstats client script 
 
 * ```JitsiMeetJS.JitsiConnection``` - the ```JitsiConnection``` constructor. You can use that to create new server connection.
 

modules/statistics/statistics.js

 // allow it to prevent people from joining a conference) to (1) start
 // downloading their API as soon as possible and (2) do the downloading
 // asynchronously.
-function loadCallStatsAPI() {
+function loadCallStatsAPI(customScriptUrl) {
     if(!isCallstatsLoaded) {
         ScriptUtil.loadScript(
-                'https://api.callstats.io/static/callstats.min.js',
+                customScriptUrl ? customScriptUrl :
+                    'https://api.callstats.io/static/callstats.min.js',
                 /* async */ true,
                 /* prepend */ true);
         isCallstatsLoaded = true;
             // requests to any third parties.
             && (Statistics.disableThirdPartyRequests !== true);
     if(this.callStatsIntegrationEnabled)
-        loadCallStatsAPI();
+        loadCallStatsAPI(this.options.callStatsCustomScriptUrl);
     this.callStats = null;
     // Flag indicates whether or not the CallStats have been started for this
     // Statistics instance