Implemented task with details

Tasks can now have details.

Also implemented a way to retrieve a task.
Improved step of task creation with due date.

The "Then" step of a the creation of a task with a due date was not
fully implemented - the due date was not checked.
Format gherkin file

Minor format of the gherkin file.
Extracted predicate out of TaskList class.

Refactoring for code lisiblity.