iOS app中跳转到iTunes store

参考：https://developer.apple.com/library/ios/qa/qa1633/_index.html

url格式为

https://itunes.apple.com/us/app/apple-store/id375380948?mt=8

把us换成对应的区域代码，id换成自己app的id，然后app中用UIApplication的openURL方法打开