In preparation for feature of removing cards from the game and adding new ones, reducing the CardGameViewController (Controller) dependence on the order (and index) of cards in the CardMatchingGame (Model).
Added additional notification from Model when cards are matched to facilitate operations such as removing cards on screen.
Played around with use of __bridge and __unsafe_unretained for converting an integer (UIView.tag) into an Objective-C Object pointer (Card *). In general not a good idea to do so. Will look at storing as type id instead of integer in next update.