Update country code validation

* Rely exclusively on `libphonenumber` for country code validation
  instead of mixing validation with `world-countries`.
* Remove ISO 3166-1 alpha-3 support as it requires extra maintenance
  and is unlikely to be that useful.
* Updated `lib/countries` to generate a list of country metadata useful
  for UI presentation and validation.
* Update google-libphonenumber@0.0.4 with metadata fixes.
Add note about non-geographical entities
Merge pull request #11 from seegno/enhancement/country-code-validation

Improve country code validation