Disable iCloud backup

儲存檔案到 Document 目錄, 會造成違反 iCloud guidelines 而被 reject.
使用以下 method 禁止 iCloud backup, 參考 Apple Q&A .

- (BOOL)__skipBackupAttributeToItemAtURL:(NSURL *)URL
{
    NSError *error = nil;  
    BOOL success = [URL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:&error];

    return success;
}