Cleanup Gemfile

Remove redundant gems like 'mail' and 'json'.
Migrate database from SQLite3 to PostgreSQL

Why? If we want to use Postgres in production, we will need to run pg in
dev and test. so we don't have to deal with issues involving differences
in SQLite and Postgres.
Create a config/config.yml file for APP_CONFIG

So we don't need to hard-code certain values into the app (and expose
sensitive stuff like AUTH tokens)
Tabs -> spaces in Twilio-related files