Set the tintColor in the auth view and modal views navigation controller on iOS 7

- The authentication controller now sets the buttons tintColor always to the default blue color. This fixes white UIWindow tintColor, since the auth view is using white as background.
- Navigation controllers for modal views now also set the tintColor of the navigationBar to the default blue color on iOS 7, if the developer did not set the `navigationBarTintColor`. This fixes white UIWindow tintColor for feedback and beta update modal views as they are defined to use the default style.