First pass contract tests for content schemas

These tests will verify that government-frontend is able to render all the
currently available examples in govuk-content-schemas. We'll use these in the
build process of govuk-content-schemas to make sure changes to the schema do not
break the government-frontend application.

Note I've had to add a Spanish translation file because the schema examples
include a Spanish translation.
Refactor schema examples to make them accessible

Now the schema examples can be used in model and controller tests as well.
Remove redundant fixtures

The tests now use the examples from govuk-content-schemas
Wire in rake task for running contract tests

This both adds a test:contracts rake task and wires it into the default test run
so that contract tests are executed as part of the default rake task.
Raise exception if can't find govuk-content-schemas

This is a bit more user friendly then the tests blowing up with a cryptic
message.
Update README with mention of schemas
Update jenkins.sh to clone govuk-content-schemas
Merge pull request #9 from alphagov/schema-tests

Use govuk-content-schemas to run contract (integration) tests
Display notice about archived documents

- Update presenter to know if an item is archived. We know it is
  archived due to the presence of an archive notice in the details hash
- Create page_title method to automatically prepend 'Archived'. In the
  future this would use an I18n method to get a localised archive title.
  However, in the current version of this code none of the archive
  notice can be translated so leaving that as a future exercise.
Downcase format type to match sentence case