[#72] Rename country.code -> country.countryCode
[#72] Adds the sites with url and search_dates to the country endpoint
[#72] Sort obi_scores on /country endpoint

I've also added tests so we make sure it'll keep working.
[#72] Rename countryCode -> country_code, to keep the coding style consistent
[#72] Fix bug when showing a country's last obi_score

We were getting the last element of the obi_scores array, but it's not
necessarily sorted. We need to sort it before doing that.
[#72] Show only active sites at country endpoint
[#72] Refactor countries.get()
[#72] Add documents to the country endpoint
[#72] Convert obi_scores' years to number
[#72] Remove country, country_code, and year from documents in /country
[#72] Add documents' states on country endpoint
[#72] Add tests for the logic that defines docs' states
[#72] Refactor /countries code

It's the first step towards making /countries have the same structure as
/country/CODE, but for all countries.
[#72] Add sites to the /countries endpoint
[#72] Add documents to /countries
[#72] Refactor countries.get() to reuse countries.list()
[#72] Require .json in the end of /countries and /country/XX

This avoids the apps that links to our API to use /countries directly expecting a
JSON, and makes us future-proof, in case we ever want to have a /countries page
that returns an HTML