Default to SQLite (over PostgreSQL)

Fixes #33 

Working on this I released that README.rst is out of date, and that lxml is not a non-trivial dependancy we added with scraping. I made #34 to address that in a similar way.
I don't think we need to support prod-requirements.txt and dev-requirements.txt in the same command.

The convention of importing a base requirements file seems to be very common.  It's the way Two Scoops does it: https://github.com/twoscoops/django-twoscoops-project/blob/develop/requirements/production.txt

I haven't seen the convention of splitting requirements and installing multiple requirements at once.   It's possible I overlooked it somewhere though.