Schema conversion

This contains 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
ah ok, is it normal to 'bundle exec' in the shebang line? 

I normally expect users of scripts to `bundle exec ./bin/combine_publisher_schema` because `bundle exec` is dependent on the current working directory (I *think*) so having it the shebang line could cause problems if you invoke it from somewhere else. I'm very open to being corrected on this if I have got it wrong.

I'm aware that I should bundle exec in the Makefile however.