add Gemfile
add method cart and receipt to UI.
add a cashier
add method add_product to UI
add manager_menu options to UI
remove un-needed lines from cart method in UI
remove until loop from main menu in UI
add a method so that a cashier has many purchases through checkouts
reverse relation methods for product and purchase classes
redo product spec for has many purchases
change customer methed to have many checkouts
add method for a checkout to have many products through purchases
add method total_cashier_checkouts to UI so that a manager can see a cashier's checkout history
finialize the cashier history in the manager menu
add method for sorting sales by product popularity