Exec code in another process to stop idiots like me redefining the Sinatra app.

This is still quite breakable. Perhaps you can use JRuby on Heroku and use
someone else's sandbox implementation (https://github.com/omghax/jruby-sandbox)
to run code in a slightly safer manner.

* Adds a runner to execute given code in a separate process
* Changes Riddle to use the external code runner