Fixed DisplayItem() for all classes in hierarchy.
Cleaned up code for << operator in AudioBook.cpp. Changed getTextForEnum() to getTextForGenre() in Book class and utilized it in DisplayItem(). This will allow up to output a string representing the genre enum rather than just a number.
Added getFileFormatText() helper function to eBook class. Will be called when using overloaded << and DisplayItem() to output a string of the file format rather than a number.