migration scripts are completely fixed (close #50)
test on downgrade in migration script in travis ci
Merge pull request #55 from sjw0687/fix_issue_50

Fix all of database migration scripts and add test on downgrade in CI
define Character and add relationships of it
Temporary workaround for import_all_modules

This should be fixed.
Return as frozen set
Test comply
Merge pull request #58 from tkiapril/import-module-workaround

Temporary workaround for import_all_modules
avoid using backref for relationship
migration script for Character and WorkCharacter
Merge pull request #56 from sjw0687/character

Add character to ontology database schema
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,