Fixed circular reference in viewer grid menu.
Fixed shutdown warning when running `gaffer test GafferTest`.
Derived all Gaffer unit tests from GafferTest.TestCase.

There was a mix of GafferTest.TestCase and unittest.TestCase base classes before. It's better if they all use the Gaffer one so they benefit from the new tearDown() method and any other future additions.
Merge pull request #881 from johnhaddon/lifetimeFixes

Python object lifetime fixes