Tests

This PR adds three tests:

- test the homepage
- test an HTML page
- test a JSON page

It also does a few other things which made testing simpler:

- Converted the app to the [modular style](http://www.sinatrarb.com/intro.html#Sinatra::Base%20-%20Middleware,%20Libraries,%20and%20Modular%20Apps)
- Added a config.ru
- Updated the .travis.yml file