Added constant DELIMITER to ReadFile class and WriteFIle class
Added constants FORMAT_DATE and FORMAT_AMOUNT; Overload two methods in WriteFile class
Added constatants to ReadFile class
Fixed bugs

1. When no list item was selected, error occured.
2. Strings are compared using == operator.
3. Error was occurred, when no customer existed in the list.
Added Search.exists(Customer c) method

This is to avoid creating duplicates when creating a new account.
Implemented PurchaseHistoryFrame
Changed number format

Comma in numeric strings caused a problem in parsing it into double
type.
Created Validation classes
Tidy up CreateAccountPrompt