Process and cache articles by period.
WIP - about to remove opendefinition license scraper
WIP. Pubmed and biomedcentral scrapers updated.
Update models and scrapers.
Catch OperationalError from both peewee and sqlite3.
Add Dockerfile. Add missing dependencies.
Move and update docker-related files.
Update developer documentation.
Update scrapers to use new syntax.
Rename examples/ dir to docs/.
Add logging. Various small fixes.
Add QUB example.
Change date_published to be a string.

Closes #17. Also use correct field name in crossref.
Implement pubmed DOI scraper.

Use transactions to improve performance and avoid purging DB if errors
occur. Add openness report (in Excel - uh... I don't feel like dealing
with escaping CSV right now). Fixes to tests. Start reorganizing
NCBI/pubmed scrapers.
Add pyflakes to development environment.
Add more logging info to report.
Remove cache image.
Log failures and improve response handling in OAG scraper.
Update DOAJ scraper.
Overhaul RCUK GTR scraper.