Add JSON tags to entity structs

This was another experiment. Kicking around the idea of using explicit JSON tags on all of the entity structs .  .  . but if we do that it seems like we should probably have explicit JSON deserialization tests (the same thing I was arguing against the other day).

Looking at some of the other Go-based API wrappers it seems like using the JSON tags is a popular convention (even when the default deserialization would achieve the same effect).