Improve tests by using more realistic examples.
Fix wrong names in test related with work.
Add association between work and person with role.
Add test for association between person and work.
Change type of role from custom type to enum type.
Adjust test_person_made_works to keep consistency.
Add migration script for work-person association.
Comment on test on people and work in conftest.py.
Add cascade behavior on relationships.
Test on cascade deletion and opposite direction.
Change migration script to merge branches.
Change type of role in Credit to EnumType.
Change migration script for credits_role.
Change quotes from double to single to keep style.
Merge pull request #10 from sjw0687/work

Add association between work and person with role.