- add persistent related code from http://www.yesodweb.com/book/persistent
- use file db instead of default memory db.
- add code generation sampe.
- add constructor error notation.
- add comment to explain sample code.
- add runMigration to create table.
- modify description of id which is not row_id.
- add uniquness.hs to demonstrate unique constraint.
- remove $ from Person constructor.