code: function init(user_id, secret, storage, callback) {
      API_USER_ID = user_id;
      API_SECRET = secret;
      TOKEN_STORAGE = storage;
  
      if (!callback) {
          callback = function() {}
      }
  
      if (!fs.existsSync(TOKEN_STORAGE)) {
          mkdirSyncRecursive(TOKEN_STORAGE);
      }
  
      if (TOKEN_STORAGE.substr(-1) !== '/') {
          TOKEN_STORAGE += '/';
      }
  
      var hashName = md5(API_USER_ID+'::'+API_SECRET);
      if (fs.existsSync(TOKEN_STORAGE+hashName)) {
          TOKEN = fs.readFileSync(TOKEN_STORAGE+hashName,{encoding:'utf8'});
      }
  
      if (! TOKEN.length) {
          getToken(callback);
          return;
      }
  
      callback(TOKEN)
  }
docs: Sendpulse API initialization
  
  @param user_id
  @param secret
  @param storage
  @param callback