set the group_name unique key on user-dedicated group creation

pboard/pboard/controllers/api.py

         loUserSpecificGroup = pld.PODStaticController.createGroup()
 
         loUserSpecificGroup.group_id = 0-loNewAccount.user_id # group id of a given user is the opposite of the user id
-        loUserSpecificGroup.group_name = ''
+        loUserSpecificGroup.group_name = 'user_%d' % loNewAccount.user_id
         loUserSpecificGroup.personnal_group = True
         loUserSpecificGroup.users.append(loNewAccount)