iOS7之前：
[[UIApplication sharedApplication]openURL:[NSURL URLWithString:@"itms-apps://ax.itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?type=Purple+Software&id=YourAPPID"]];
iOS7之后：
[[UIApplication sharedApplication]openURL:[NSURL URLWithString:@"http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?id=YourAPPID&pageNumber=0&sortOrdering=2&type=Purple+Software&mt=8"]];