bug fix: Changing the role of a project member/group should not remove it from the project.

- Changing the role of a project member should not trigger a deletion.
- The project member should however be destroyed as soon as all inherited roles have been removed and it has no (direct) roles assigned to it.
- Test have been added to project core_tests.
Merge pull request #45 from opf/fix/member_role_change

bug fix: Changing the role of a project member/group.