add preferredStatusBarStyle

If you have dark navigation bars, then the status bar text is black-on-black.

    [[UINavigationBar appearance] setBarStyle:UIBarStyleBlack];

By adding `preferredStatusBarStyle` to the split view controller, callers can specify light content.

    BDBSplitViewController *splitViewController = [[BDBSplitViewController alloc] init...]
    splitViewController.preferredStatusBarStyle = UIStatusBarStyleLightContent;