Added creation time and creator user to groups, memberships and permissions entities

gms/src/main/java/it/inaf/ia2/gms/manager/GroupsManager.java

...a/it/inaf/ia2/gms/manager/InvitedRegistrationManager.java

.../main/java/it/inaf/ia2/gms/manager/MembershipManager.java

...main/java/it/inaf/ia2/gms/manager/PermissionsManager.java

gms/src/main/java/it/inaf/ia2/gms/model/GroupNode.java

gms/src/main/java/it/inaf/ia2/gms/persistence/GroupsDAO.java

gms/src/main/java/it/inaf/ia2/gms/persistence/JoinDAO.java

...main/java/it/inaf/ia2/gms/persistence/MembershipsDAO.java

...main/java/it/inaf/ia2/gms/persistence/PermissionsDAO.java

...n/java/it/inaf/ia2/gms/persistence/model/GroupEntity.java

...a/it/inaf/ia2/gms/persistence/model/MembershipEntity.java

...a/it/inaf/ia2/gms/persistence/model/PermissionEntity.java

gms/src/main/java/it/inaf/ia2/gms/service/GroupsService.java

.../main/java/it/inaf/ia2/gms/service/GroupsTreeBuilder.java

...main/java/it/inaf/ia2/gms/service/PermissionsService.java

gms/src/main/resources/sql/init.sql

gms/src/main/resources/sql/upgrade.sql

.../inaf/ia2/gms/manager/InvitedRegistrationManagerTest.java

.../java/it/inaf/ia2/gms/manager/PermissionsManagerTest.java

.../test/java/it/inaf/ia2/gms/persistence/GroupsDAOTest.java