Provide geographical_area_id instead of iso_code.

Geographical area id is not always an iso code. It's iso code for
countries, but otherwise it's geographical area group id. So it should
be up to client app to decide when to call this an iso code.
Merge pull request #53 from alphagov/rename_iso_code_on_geographical_area

Provide geographical_area_id instead of iso_code.