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.

adds hashable field to group and constituency, get_or_create for group is case insensitive, with fingerprint it will be bulletproof

It's not possible to have a ForeignKey pointing on an abstract model,
so we have two options to keep Representative: keep in asbtract, and make
mandatory to define one and just one class that inherit from it ; have it
non abstract, and so having the possibilty to create serveral classes that
inherit from Representative, but so it means that all requests will have a
JOIN between the two tables