Checkpoint.
Finally have a handle on the transition.
* Had to take snapshots of views instead of manipulating them directly because of constraints.
* But this also made for cleaner abstraction (could just hide actual views and animate their snapshots)
* Detail view must be in a scroll view, because that’s the only way iOS7 can extend content beneath the nav bar.
* You’d think topLayoutGuide would help, but it doesn’t (http://stackoverflow.com/questions/20312765/navigation-controller-top-layout-guide-not-honored-with-custom-transition)