Expose country list with identifiers via API.

This is needed to build a list of countries for import/export filtering
functionality. Same thing as in EU Taric site, see Country of origin/destination in http://ec.europa.eu/taxation_customs/dds2/taric/taric_consultation.jsp?Lang=en

Related to Pivotal ticket https://www.pivotaltracker.com/story/show/36718601.

There will be a frontend counter part for this, but I want to see the final design for merged search form first (https://www.pivotaltracker.com/story/show/38329451).