Since this was a relatively simple web app I decided to use a small framework Cramp, which I've used in the past and is easy to make simple asynchronous web applications from. One disadvantage of doing this is the lack of a proper MVC layout, which might have made rendering the views and storing the data a little simpler but on the whole there were not many issues.
I've separated out the logic to create the hashkey and also to check the generated response. There's also some logic here to generate the parameters from a Ruby hash. Seprating out the concerns allowed me to unit test these parts of the app.
So far I have only written unit tests but it would make sense to write some integration tests for the application.