Improved removal of egg-related files in Makefile

The current implementation does not remove any ``.egg`` files that ``tox`` adds to the root folder when it installs dependencies defined in ``setup.py``. I've amended the script to remove any and all ``.egg`` files when cleaning.

I also have a single-module Python project, which has a ``setup.py`` which looks like this:

    setup(
        [...]
        py_modules=[
            'package-name',
        ],
        package_dir={'':
                     'src'},
        [...]
    )

Though perhaps a bug with ``setuptools``, this adds the ``egginfo`` folder to the ``src`` directory, rather than to the root directory. The script now also removes any and all ``egginfo`` folders.