iOS8以前：
UIApplication *app = [UIApplication sharedApplication];
app.applicationIconBadgeNumber = num;
iOS8及以后：
UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTypes:UIUserNotificationTypeBadge categories:nil];
[[UIApplication sharedApplication] registerUserNotificationSettings:settings];
需要先设置它，才行。而且，需要注意一点，iOS8以前，不能带它，iOS8及以后必须带它。
所以现在再用它的时候，需要先判断系统版本号：
float version = [[[UIDevice currentDevice] systemVersion] floatValue];
if (version >= 8.0) {
UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTypes:UIUserNotificationTypeBadge categories:nil];
[[UIApplication sharedApplication] registerUserNotificationSettings:settings];
}