"[Drop] That Weight" or Updated dev database and setup docs

Two changes:

- Revised documentation in `doc` folder. It was a little out of date and the file names weren't super clear.
- Made SQLite3 the development and test database. This drops a significant part of the setup for developers and makes it more lightweight/possible for running unit tests in isolation. Staging and production environments are still configured to PostgreSQL (i.e. it'll use PostgreSQL for `rails server -e staging` or `rails server -e production`).

@codesliced @markGiansanti You're both out today and Monday, so I'm going to merge this in. Wanted to tag you just so that you're aware of it and I'm not sneaking it in.