Add association between work and person with role.

`Credit` in `work.py` is table for the association between the work and the person. the association table has extra column `role` is enum type (it could be null if not known). The association describes that the person participated in making the work. Just `Team` table was not sufficient to find who participated in making the work. `role` column denotes he/she's part in making the work, e.g., artisit, author and editor.
Why not `EnumType` + `enum.Enum`?