Default to SQLite (over PostgreSQL)

[macro1]

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Working on this I released that README.rst is out of date, and that lxml is not a non-trivial dependancy we added with scraping.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.

[treyhunner]

I think I would prefer to add a -r requirements.txt line here instead of listing two requirements files in the pip install.

[macro1]

Requirements can only be specified once, even if they don't conflict. If I add the general requirements here, it will prevent this requirement file from being combined in the same install command as dev-requirements.txt.

What we could do is have a folder with the different types of requirements, and these top-level files would just combine them.

Edit: What I meant is specific package requirements can only be specified once, you can use multiple requirement files in the same installation command as I've done in both the CI and deploy configuration.

[treyhunner]

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.