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Working on this I released that README.rst is out of date, and that lxml is not a non-trivial dependancy we added with scraping.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