Calendar - Add an event
Event - test improving

Changed a string the Hatter to the Mad Hatter
Calendar - Display events by date ascending
EventRepository (refactoring)

Events are provided by a repository
Calendar - Cannot add an overlapping event

Throw OverlappingException otherwise.
Calendar - Refactoring
Calendar - Test refactoring
Rename unit tests
Calendar - Cannot add an event in the past
EventRepository - Do not return past event
EventRepository - Tests refactoring