Speedup countries

[felixbuenemann]

• Use @countries.includes(:states) to avoid n+1 queries on states
• Drop the has_one :country on the StateSerializer and replace with attribute :country_id to avoid n+1 queries for the country
• Including all the countries might not be needed in most cases, so it's possible to add a query param to filter by country codes (eg.: api/ams/countries?isos[]=DE&isos[]=CH&isos[]=AT)

Doing the first two brings down load time from > 30s to ~5s, the last example takes < 100ms.

Related to issue #11.