Change show tryout to new tryout

- Use first quiz for tryout quiz
- Remove unused request spec for tryout
Change vagrant shared folder to use nfs
Add single table inheritance to table user
Add model student with user inheritance

- Add model student that inherit user
- Refactor user behavior to shared examples
Add model parent with user inheritance
Add student parent relationship
Change should to is_expected for spec
Add tryout model
Add examine tryout score
Remove hstore extension on postgresql
Add sample tryout questions
Change secret key base type case