Use more common words, and words that differ more in their declension
Exercise framework extracted and tweaks for mobile

* The exercise framework has been extracted into its own project, included here in js/lib/exercises.min.js. The code specific to this app (Finnish exercises) now lives in js/app.js.
* There are some CSS tweaks to make it a little nicer to use on small screens, such as mobile.
* The HTML has been simplified.