[#6] Add support for group resource

Pull request to resolve a part of #6, with following additions (strongly inspired/copied from the user implementation #47):
  * support of 'group' resource
  * some minor code cleanup found by the way

IMO following points are still open:
  * implement matchers for missing user/group resource actions: `:modify`, `:manage` (`:lock`, `unlock`)
  * provide more examples / scenarios ?

Thank you in advance to answer points above during the review... I'll be glad to improve this pull request with your comments!