Fallback to the UIAppearance value for barTintColor

Without this change, the overlay fade effect does not work for applications that only specify barTintColor using the appearance proxy.

Currently, the barTintColor would have to be specified explicitly on the UINavigationBar instance referenced by self.navigationController.navigationBar. With the modifications I've made, the barTintColor will check for the UIAppearance value on UINavigationBar for the barTintColor before issuing the warning "no bar tint color set".