DOC: Add example Dockerfile for running a Postgres backend.
DOC: Add instructions to README.md
DOC: Tweaks to comments in initdb.sh.
DEV: Split postgres example into separate db and hub containers.
DOC: Update the README for postgres example.
DOC: More tweaks to postgres example.
DOC: Add note about default username/password configuration.
DEV: Use docker links to set password environment variable in the hub.

Also, yell at people if they use the default password.
DOC: Fix typo in postgres example message.
DOC: Fix more typos in warning messages.
Merge pull request #87 from quantopian/postgres-example

Postgres example using docker