Add tests/compat.py for test-only imports

This fixes the issue discovered in https://github.com/spulec/moto/issues/151 as part of #2437 by making sure we don't introduce new dependencies into our `boto/compat` module. New tests should use `tests.compat` instead for `unittest` and `OrderedDict`.

The majority of unit tests already inherit from `AWSMockServiceTestCase` so there was no need to update those once `tests/unit/__init__.py` had the correct import from `tests.compat`.

@jamesls, @spulec please have a look.