Make it work with https everywhere

Sorry for bothering you again; it looks like I ~~spoke~~ sent a pull request too soon.

I tried to use the examples with [HTTPS Everywhere](https://www.eff.org/https-everywhere) enabled and they still didn't work due to mixed content being blocked. So now I fixed (and tested) all of them to make sure they work over https as well as over http.

The only code change is that the `iframe` now always points to the `index.html` file of the snapshot instead of just the folder. This is because Github redirects folders from https to http. All other changes are just removing the http protocol prefix.

Notice I did touch neither the `example.json` nor the `compileReadme.js` which seems to be generating it, simply because the `runUrl` property of the JSON objects is never used.

Oh, and thank you again for this awesome tutorial!