add warning when reinitialize

Author: aisk

lib/utils.js

@@ -128,6 +128,9 @@
    * @param {String} applicationKey Your AV Application Key
    */
    AV._initialize = function(applicationId, applicationKey, masterKey) {
+    if (AV.applicationId !== undefined) {
+      console.warn('AVOSCloud SDK is already initialized, please don\'t reinitialize it.');
+    }
     AV.applicationId = applicationId;
     AV.applicationKey = applicationKey;
     AV.masterKey = masterKey;