Fix creation and completion dates

Hi,

There are some major changes in this PR.
created_on and completed_on are the two new attribute which represents the dates. The original 'date' is aliased to created_on for backward compatibility - I did not want to remove it.
~~Task#due? is always returning nil (could be false) because due date is not implemented (I will send a new pull request with that).~~

- [x] implement logging
- [x] implement #overdue? due_on
- [x] improve logger