Replace ao3 downloader library

[annapods]

The libraries or code used for working with ao3 are cumbersome and don't communicate well. It would be great to have one, single, coherent API for all.

Here's what is used at the moment:

• downloading the work(s): ao3downloader in html_downloader.py
• parsing it: homemade regex in html_extractor.py
• posting: ao3_poster in ao3_drafter.py
• data handling: project_metadata.py, while not an ao3 functionality, is heavily involved in the process

[annapods]

ao3-poster code has been integrated and adapted in ao3_drafter.py