Add a class extension in which make the start and stop proprieties readwrite
Add initializer to the GameResults class
Add getter for the duration property
Add setter for the score property
Add method that returns the GameResults object as a property list
Add a synchronize method that writes the results to the standard user defaults
Add custom initializer that receives a plist and creates a GameResults object from it
Implement the AllGamesResults method that returns an array with all the results of all games
Create the class methods that return a sorted array of all the results of all the games
Create the comparison selectors used to sort the array of games results