fix(database): add referential actions in purchase, and add default v…

[imbensonchiu]

fix issue #17, #18, and #14

@togimoto

[imbensonchiu]

Sorry for getting the content of DB_Final.sql wrong, I mistakenly thought it was the backup file of our database. Now, I create two folders:

• ./db_backup for storing the backup file of NTU BOOK's database
• ./query for storing the SQL query file

Also, I've added the default value as well as renaming timestamp in purchaserequest to requesttimestamp

[imbensonchiu]

@togimoto: After merge, please check #25