Remove comments

Introduce isOverdraft method
- And remove one of the comments

Move isOverdraft method to Account class
- Having the method in the Customer class smells like Feature Envy

Introduce CustomerFactory
- Remove one of the constructors, leave only the generic one, handle details in the factory
- No Factory Method pattern because we plan to make a class hierarchy
- The constructor can only be accessed only from the package
- Refactor the tests to use the new factory

There still are some comments but we'll be deal with them later