Expose coversheet node as part of public Flow API

Previously this was private, but it's useful to some clients, see:
 - https://github.com/alphagov/smart-answers/blob/master/lib/smartdown_adapter/flow_registration_presenter.rb#L44

Also we expose questions, question pages and outcomes, so we should
do the same for covers.
Merge pull request #38 from alphagov/make-coversheet-public

Expose coversheet node as part of public Flow API