Added a LastLetterChoosable variable to make ranges of guess options simpler
We now check if they want to continue playing (though this question can't be asked yet)
Made CodeItems an array in CodeSet, and added some functionality to CodeSet to convert from strings
Well, now we're checking guess validity pretty well.
Using ArrayLists like C/C++ Arrays is a bad thing.  Let's do it the C# way.  Oh, and increment X in while loops.