Implement the Iterator interface for the parser

[wadmiraal]

This PR contains 3 changes (sorry, I just didn't plan that very well when making the changes; if you want, I can cherry-pick and split it into 3 separate PRs):

1. Add a unit test for non-existent files when using VCardParser::parseFromFile() (throws an exception)
2. Add a new method getCards(), which gets all cards. This is to complement the getCardAtIndex() method (which is great, but not sufficient).
3. Implement the Iterator interface to loop over parsed data. Let me detail this last one.

Implement the Iterator interface for the parser class. This will allow parsed VCard databases to be easily iterated over. Example:

$parser = new VCardParser($vcardDb);
foreach ($parser as $i => $card) {
    // Do stuff with card.
}

This is much more DX friendly than the simple getCardAtIndex() and getCards() methods. Added unit tests accordingly.