Fix -view(Will|Did)(A|Disa)ppear: being called twice.

- This happened under iOS 6 where the “new” view controller containment
stuff exists. In other words, both AKTabBarController and iOS would
send these messages.
- Due to this change the deployment target has been bumped to iOS 6.
- Found this out due to a bug where the previous selected view
controller was being set to the new view controller.


Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>