Created Model and added Card interface and implementation.

Added new class `Card` to Model.
Created Model group in Xcode project navigator and as a sub-directory to
project on file system.
Added PlayingCard interface and implementation.

Added PlayingCard class to model.
Added Deck interface and implementation to model.

Added class to represent a deck of cards to the model.
Added PlayingCardDeck interface and implementation.

Added class for PlayingCardDeck.