representative_post_save is gone, thanks to memopol rewrite.

It was replaced with representative_pre_import, which allows skipping
some meps.

A mixin to encapsulate pre-fetching logic for Representative country and
main_mandate.

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.