Add Country pseudo-model.

This is a temporary thing driven from a yaml file, and will eventually be replaced by an API.
Add handling for travel-advice artefacts
Remove reference to TagRepository

This has been removed from govuk_content_models
Unify travel-advice artefact serving

No longer need a separate 'route'
Add comment explaining travel-advice special case
Add some more tests around titles and descriptions.
Merge remote branch 'origin/master' into add_travel_advice

Conflicts:
	Gemfile
	Gemfile.lock
Update to released version of govuk_content_models

Using version with TravelAdviceEdition
Merge remote branch 'origin/master' into add_travel_advice

Conflicts:
	Gemfile.lock
	govuk_content_api.rb
Merge pull request #63 from alphagov/add_travel_advice

Add travel advice