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 hierachy
- The constructor can only be accessed only from the package
- Refactor the tests to use the new factory
Merge pull request #2 from danpersa/02-remove-comments

Remove comments