Add 'rspec-rails' to development gem group

This means that specs are run when `rake` is invoked.
Add app-specific rspec configuration
Port browsing-a-local-manual to rspec
Port 'view-a-manuals-updates' to rspec
Remove cucumber from project
Introduce domain layer for specs
Merge pull request #74 from alphagov/replace-cucumber-with-rspec-features

Replace 'cucumber' with 'rspec' features