Improve build procedure, Add tests

[sshine]

Hi @boxyoman!

Thanks a lot for your effort so far!

I'd like to contribute to make this Servant-based example stable.

Collecting what seems to be loose ends in gothinkster/realworld#73:

• Complete API
• Discuss what must be tested
• Consider Windows / MacOS support (I believe we can use GitHub actions for a build matrix that cover both) -- I haven't investigated why this is difficult, but if it's related to deployment of Postgres, perhaps we can discuss this separately.

Unfortunately, following your build instructions (with zero Nix experience), I get:

$ nixops deploy -d realworld --force-reboot
...
error: attribute 'version' missing, at /home/simon/.nix-defexpr/channels/nixpkgs/nixos/modules/services/databases/postgresql.nix:306:42

Probably the postgresql.nix file had version constraints added since you last modified this project? Adding --show-trace does print out a whole lot more, but I'm not able to decipher what file to touch in order to resolve the missing 'version' attribute.

Also, since this is apparently a bit difficult to get working, perhaps we should consider multiple installation methods? E.g. via cabal/stack also. Something that eckyputrady/haskell-scotty-realworld-example-app (which is also mostly done and abandoned) is Heroku deployment, which lowers the bar for getting Postgres up and running. @anishkny was suggesting a Dockerfile.