Schema conversion

Defining the 'publisher' schema in terms of three separate parts (metadata, details, links) will allow ownership of each of those files to reside in the appropriate place, namely:

  - metadata is owned by content-store
  - details and links are owned by the corresponding publishing applications

We anticipate that the partial files will be stored in govuk-content-schemas as a central repository of the files, but that the 'master' copies of them would reside in the owning repo (content-store or the publishing application). 

This pull request adds a script for combining `metadata.json`, `details.json` and `links.json` schemas into a single overall schema.

https://trello.com/c/QdWO5hjC/82-automate-schema-verification

- [x] update readme
- [x] tweak `#embed` to describe `excluded_top_level_keys`
- [x] remove `FixtureHelpers`