Fix viewWillAppear bugs

`viewWillAppear` had two bugs

a) missing a call to super
b) when you used `SBSegmentedViewController` in e.g. a tab bar controller, when you switched away from the `SBSegmentedViewController` tab and came back, `viewWillAppear` was called again on `SBSegmentedViewController`. This caused the current view controller to go back to the default selected index.

To solve b), I added a `BOOL` to see if `SBSegmentedViewController` had appeared yet. Typically, I would move this logic to `viewDidLoad`, but `SBSegmentedViewController` does not yet have the correct frame in `viewDidLoad` to place the child view controller.