- Added update(Book) on BookFactory.java

- Declared users as ArrayList
- Fixed false declaration method of newUser(String, int)
- Renamed new and old to newUser and oldUser due to ‘new’ unusable

User.java
- Added getId()

UserFactory
- Added simple constructor (to be modified further)
- Added newUser(), getUser(String), getUser(int) for interaction with
the List<User>
- Added update(User, User) to update/replace an item in the List