Updated to Swift 4.2

Slightly tidier way of adding observers.

Fixed an update issue when renaming a timezone.

Major overhaul of implementation.

Refining the implementation a little.

Removed debug example that doesn't match current code.

Noted Swift 4 requirement.

Avoid reloadData from stomping on contentOffset changes.

Use a segue for detail presentation – it's less code and it more closely matches the non-view-state version.

Minor fix for model change affecting view state.

Update the presentation status on collapse.

Fixed some transition bugs and included debug code (commented out) that trigger one of the bugs for demonstration.

Added a README

Merge branch 'timetravel'

Applying updates from timetravel branch.