updated README to start pull request for assignment 1
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.
Added private property for deck to View Controller.

Added private property (deck) to CardGameViewController. Override on getter to
lazily instantiate PlayingCardDeck when needed.
Flipping cards now shows random playing card from deck.

Modified storyboard to show card back initially.
Modified touchCardButton to draw a random card from a deck of Playing Cards and
display it. Deck is hidden after last card is shown.
Merge pull request #1 from zanshin/assignment/one

Completed all six required tasks for assignment 1.