Refactor class into a module

These edits started with me trying to fix this error I got during slug compile on Heroku:
````
       There was a LoadError while evaluating schiphol.gemspec:
       no such file to load -- zip/zip from

/tmp/build_26b8yso2vcet4/vendor/bundle/ruby/1.9.1/bundler/gems/schiphol-
553c543/schiphol.gemspec:3:in `<main>'
       Does it try to require a relative path? That's been removed in
Ruby 1.9.
````

To fix this, I had to not require 'schiphol' in the gemspec. So I made a `lib/schiphol/version.rb` per [http://timelessrepo.com/making-ruby-gems] and then encountered some problems because Schiphol was a class instead of a module. The class is never instantiated so a module is appropriate, and the last commit in the set makes that change.