Add new storage type models logic to work with SQL server

You have relations in your models. Why did you choose NoSQL? Dont you want to create SQL store type just to try? I can do it also. I have created today docker env https://github.com/google/peoplemath/pull/42 so I can add PostgreSQL container with adminer.

@amdw reply
Regarding the choice of data store: a NoSQL database seemed like a good fit because the Period data structure is quite deeply nested (not having to do joins potentially brings some performance gains), and we have quite simple querying needs. The lack of schema makes it very easy to add new fields - you just modify the Go structs and everything just works.

The nice thing about Cloud Datastore specifically is that there is quite a generous free tier, meaning that you can run small instances on App Engine without paying anything, which is not the case even for the smallest SQL database (as Cloud SQL instances require a dedicated VM).

That said, I can well imagine that some users would prefer to use a SQL data store, and I'm quite happy to accept a new version of the StorageService that uses a SQL database instead, perhaps using Gorm or similar. The only thing is that I'm not likely to use it myself, so I can't promise to invest time in maintaining it.