Added Meetup requirements to the README
#46: Better API failover
#46: fixed indentation and added some documentation
Merge pull request #74 from rob-erwt/better_api_fails

#46: Better API failover
[fix] issue #45 - make the group_urlname configurable

- abstracted the AbstractService::__constructor()
- abstracted common methods: AbstractService::getApi(), AbstractService::getCache()
- no changes to .gitignore

Conflicts: src/AmsterdamPHP/Bundle/MeetupBundle/Service/EventService.php