First steps with ScalaTest

I've partially done ParserSuite, but in process I had some questions that should be cleared before accepting (not yet ready) PR.

* Maybe we should store those large strings and arrays of strings in files or serialized? That's not a performance hit since that doesn't affect runtime and that's more elegant and concise.
* Second, I'd like to suggest to replace Array of `stopWords` with Set because lookup on Set is of effective constant time and we get `distinct` for free. I guess, that could improve overall performance.
* And if you agree with that, then maybe I should look for more appropriate data structures in other places as well?