Implement empty class files for each component

Why is this change necessary?
- To start coding in parallel, i.e. each team member takes one component
  and implements it.
- The four components/classes (UserInterface, Logic, Database,
  GoogleCalendarManager) forms the architecture for our project
  TaskBuddy.

What side effects does this change have?
- We have to start coding and implementing all the methods of our
  classes.