Reordering screens in tab bar controller so that Star comes first, followed by Demo, followed by Problems.
Changing static variable in ProblemAnimations into a property instead.
Improving documentation and organization of Star screen
Includes correcting a couple of typos on the Demo screen
Doing some upgrades to graphics and changing simplifying file project organization
File reorganization part 1: Renaming SALetterLabel to DemoLetterLabel to make it clear that this is part of the Demo screen.
File reorganization part 2: Reordering the files in the project so that they follow the same order as the screens in the tab bar controller.
File reorganization part 3: renaming "Demo" classes and labels to "Card" classes and labels.  Removing properties and variables that referred to "letters" and changing those references to "cards."
File reorganization part 4: Renaming "Demo" files to "Card" files
File reorganization part 5: Final code changes due to renamed files.
Upgraded README file
Minor upgrades to the CardViewController.xib