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
This method and `build_string_properties` are functionally identical. How about a single generically named method that could be used in both cases? I don't have a good suggestion for that though...