Entry resource CRUD

www.agileplannerapp.com/boards/173808/cards/7897

As a search and browse developer
I want to be able to discover the `content_ids` of topics
so that I can tag content to those topics

includes and `Entry` model, and controller actions to be able to submit entries, and retrieve entries belonging to a particular format.

i'll work on updating README and creating a `/doc` with input output examples if this looks good.
I'd suggest we make this match the [validation used in content-store](https://github.com/alphagov/content-store/blob/master/app/validators/uuid_validator.rb).  It further restricts it - not all positions can be any hex digit.