iOS 8应用内跳转到系统设置

iOS 8以后，苹果重新允许从应用内跳转到系统设置，在设置前须判断当前手机系统是否能够跳转到系统设置：

if ([[UIApplicationsharedApplication] canOpenURL:[NSURL    URLWithString:UIApplicationOpenSettingsURLString]])
{
     [[UIApplication sharedApplication] openURL:[NSURL URLWithString:UIApplicationOpenSettingsURLString]];
}