Added a simple test prolog program to use with the system temporarily

Added "shopping.pl", which is a prolog program that predicts whether a
shopper will purchase a particular food item based on whether that
item is seasonal and whether the shopper purchased the item previously
during the same season. The program relies on two text files.  The
file "eventlist.txt" tracks purchases and the file "newfood.txt" is a
list of food items to be added to the database once the prolog program
"shopping.pl" is run.