Added the method #all_translated(locale) that returns a list of translated country names.

- The returned list is only an array of country name strings
- The returned list is alphabetized in the translated language
- Countries that are missing locales are dropped from the list

I saw someone requested a feature like this: https://github.com/hexorx/countries/issues/110

And someone asked a question on StackOverflow requesting this functionality: http://stackoverflow.com/questions/22366065/translate-array-of-countries-from-countries-gem-in-rails/22366129#22366129