Issue with view-controller based statusbar style

[pronebird]

Would be terrific to have support for view-controller based statusbar style. More or less statusbar will consistent across controllers, so statusbar style option can be added to theme plist for the beginning.

Even for basic support, it could be easy just to add the following thing to controllers to make things work:

- (void)preferredStatusBarStyle {
    return self.currentStatusBarStyle;
}