Rubocop

I added rubocop with some configuration to the repository, and made sure all issues are fixed or ignored.

Now when you run bundle exec rubocop it automatically checks style. I think this way we can more easily enforce consistent styling in the repository, and also catch bugs (I accidentally first based this upon an old branch, and it actually catched some faulty specs).

I'm well aware that some style is very personal, so if you disagree with anything, please say so! All cops can be individually be disabled and configured. I already disabled the single quotes configuration (since that's rather silly imho).