DateTimeImmutable support

[dereuromark]

Jsvrcek\ICS\Model\CalendarEvent::setStart(): Argument #1 ($start) must be of type DateTime, DateTimeImmutable given

It should also in general accept the interface IMO (also DateTimeInterface instead of concrete DateTime)
so also other libraries work:

• Carbon
• Chronos

[jasvrcek]

@dereuromark you are right, it would be better for the type hint to be DateTimeInterface. Would you mind making a pull request?