Shaded relief

Unfortunately, this pull request mixes-up changes I made to catch-up with your work, and the changes I made to 1. Change the scale of the country borders base map, and... 
2. Add the shaded relief layer as an overlay, using as compact a TopoJSON dataset as I could create quickly.

You should be able to create just the diffs you need, or even copy and paste the relevant additions and changes, but I'm happy to rebase and re-apply my changes, to get a clean merge in a new pull request.

Let me know which you'd prefer.