add alembic patch: user and credentials
Fix original slug not nullable
Merge pull request #63 from tkiapril/tvtropes-model-fix

Fix original slug not nullable
Merge pull request #62 from item4/db/social-login

Add alembic patch: user and credentials
count_by_classes
create table for wikipedia entities like artists and works. in order to align with tvtropes entities.
adjust test
adjust alembic auto generate files.
remove unused alembic version
single table inheritance design
multi-level inheritance
single table inheritance with test
adjust isbn to String,
crawl class and load dbpedia entities regardless HTTPError
merge alembic scripts
set retry limit when HTTPError
changed location of get_wikipedia_limit():
added last_crawled
enhance redability
last_crawled nullable=False