Reload the app on code change.

I used sinatra-contrib's Sinatra::Reloader in the end.

The first thing I tried was [rerun](https://github.com/alexch/rerun) as [recommended by Sinatra](http://www.sinatrarb.com/faq.html#reloading). But, it would only reload on the first file change and then ignore all changed afterward.

Then I tried Rack::Reloader, but it didn't seem to work at all.

So, finally, I just followed the final bit of giving up "[if you still want in-process reloading](http://www.sinatrarb.com/contrib/reloader)" instructions. They did the trick.