Added more Windows-specific PostgreSQL installations steps

Some steps were missing, for a complete installation of PostgreSQL on Windows:

- The installer doesn't add `psql` to `PATH`. I've added a step of doing this using the Command Prompt (I think it's easier to explain than the GUI way... what do you think?).
- `psql` needs explicit username and password to start, so `psql -U <username> -W` is needed.
- psycopg2 needs a compiler to pip-install. I've added a suggestion to use https://github.com/nwcell/psycopg2-windows instead. Is that all right?