add a list of group by type

refs #53

The new route is '/groups/bytype/{_type}.{_format}'

Example : '/groups/bytype/moderator.json'
Merge branch 'master' of git.champs-libres.coop:WikiPedale
refactoring src/Progracqteur/WikipedaleBundle/Controller/GroupFrontendController.php : psr-2 convention
UI #53