Database things

Hey there!

I noticed that there's no mention of the requirement for Postgres anywhere in the read me. (There's actually an issue open right now due to this - basically a package is required by Django for Postgres that isn't specced out in requirements.txt)

This PR would do three things:

* Add the `psycopg2` package to requirements so those get installed as part of the instructions
* Modifies the readme to this effect, and also:
* Adds a note there and in config.py that SQLite3 is a viable alternative. I've been running this app for a week or so now with SQLite just for use in an internal group, and performance has been great with no hiccups at all!

Hope this helps! Thanks for the awesome app!