remove_countries_without_group and remove_groups_without_mandates are
useful to clean the database before dumping test fixtures

It includes latest research by Arnaud Fabre but I ended up squashing all
commits and adding my own fixups in this commit.

Also:

- the import command is tested,
- code was PEP8'ed
- untested code was removed,
- duplicate country Israel was removed and migration was added,
- requirements were deleted in favor of proper setup.py.

It can be any kind of representatives not only eu mps