Changed size of card images

Changed the sizes of the cards to fit the new background. Have also
removed unnecessary files from assets folder
Incorporated background into gui_blackjack

The program now includes the background image
Changed Hand class

The class no longer automatically adds two cards but includes the
methods twist and add_card. The former gets a card from the deck whilst
the latler manually adds a card specified by a parameter.
Created DealerHand class

Created the class which inherits from Hand because the dealer has some
specialist functions.
Displays hand to user in graphic form

The user can now see his own hand as well as the dealer's. At the
beginning only the dealer's top card is shown.