Group urlname configurable [issue #45]

Fix for issue AmsterdamPHP#45
- making the Meetup group URL name configurable
- abstracted the common behavior of the `*Service` classes

`.gitignore`
 - exclude IDE files (PhpStorm)
So far the classes all align pretty well so this works. However its good to note that this is risky in some cases and may lead to some backfire. But in this case its ok.