GMS client improvements

.../it/inaf/ia2/gms/client/call/GetMemberEmailAddresses.java

...n/java/it/inaf/ia2/gms/client/call/GetUserGroupsCall.java

...a/it/inaf/ia2/gms/client/call/GetUserPermissionsCall.java

...main/java/it/inaf/ia2/gms/client/call/ListGroupsCall.java

...in/java/it/inaf/ia2/gms/client/call/RemoveMemberCall.java

...ava/it/inaf/ia2/gms/client/call/RemovePermissionCall.java

...n/java/it/inaf/ia2/gms/client/call/SetPermissionCall.java

...in/java/it/inaf/ia2/gms/client/model/GroupPermission.java

...rc/main/java/it/inaf/ia2/gms/client/model/Permission.java

...ain/java/it/inaf/ia2/gms/client/model/UserPermission.java

...c/test/java/it/inaf/ia2/gms/client/BaseGmsClientTest.java

.../test/java/it/inaf/ia2/gms/client/call/AddMemberTest.java

...t/java/it/inaf/ia2/gms/client/call/AddPermissionTest.java

...est/java/it/inaf/ia2/gms/client/call/CreateGroupTest.java

...est/java/it/inaf/ia2/gms/client/call/DeleteGroupTest.java

...t/java/it/inaf/ia2/gms/client/call/GetUserGroupsTest.java

.../it/inaf/ia2/gms/client/call/InvitedRegistrationTest.java

...st/java/it/inaf/ia2/gms/client/call/RemoveMemberTest.java

...ava/it/inaf/ia2/gms/client/call/RemovePermissionTest.java

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