Add an NSMutableArray to hold all the flipped cards

The propriety is lazy instantiated.
Add flipScore propriety to hold the partial score on a flip
Add gameStatus propriety

gameStatus tells the user if there was a match (1), a mismatch (-1), or
just a flip (0).
Use flipScore to update the actual score
Modify score constants
Add cards to flippedCards array

If just a flip add just one card, otherwise add all the cards that are
face up and playable.
Set gameStatus to the appropriate values for the different situations

Set to 0 for just a card flip, to 1 for matching cards, and to -1 to
mismatching cards.
Add whitespace changes