xxxxx.xxxxx.adgroups.item.exist
        ===================================
        判断在一个推广计划中是否已经推广了一个商品

def item_exist(self, campaign_id, item_id, nick=None):
        '''xxxxx.xxxxx.adgroups.item.exist
        ===================================
        判断在一个推广计划中是否已经推广了一个商品'''
        request = TOPRequest('xxxxx.xxxxx.adgroups.item.exist')
        request['campaign_id'] = campaign_id
        request['item_id'] = item_id
        if nick!=None: request['nick'] = nick
        self.create(self.execute(request))
        return self.result