document.py: Uses dummy abstract class method decorator on Python 2.
converer.py: Changes converts to inherit from scrapers.
scraper.py: Refactors link scrape functions.

ScrapedLink renamed to LinkInfo. LinkInfo adds new element and
value_type attributes. Scrape functions changed to class methods.
database.py: Fixes urltable contains.
converter.py: Implements HTMLConverter.
web.py: Fixes files not written.
database.py: Fixes __iter__().
converter.py: Detects link_type if not provided.
Implements --convert-links,--backup-converted.
changelog: Updates latest entry (files not written, --convert-links).

Closes chfoo/wpull#4. Updates terse_options.rst