[IDM] - Supporting Groups with the same name but different parents.

The stores must support groups with the same name according to the following rules:

Groups with the same name must have different parents.

identityManager.add(new SimpleGroup("groupA"))
identityManager.add(new SimpleGroup("groupA", new SimpleGroup("groupB")))
identityManager.add(new SimpleGroup("groupA"), new SimpleGroup("groupC")))

The method getGroup(String) from the IdentityManager interface should be changed to expect a path instead of the group name:

identityManager.getGroup("/groupA/groupC") // should return the groupC with the groupC as the parent group.

The method getGroup(String, Group) from the IdentityManager interface should be changed to accept null as the Group argument to return groups without parents (root groups).

identityManager.getGroup("groupC", null) // should return the groupC.