Added shop functionality, some background changes

Added shop functionality
 - buying a weapon will give you that weapon for the next level 
 - buying a weapon you already have will increase the ammo
 - buying a fedora increases your health
 - the new weapon and ammo is only good for the next level played
 - the health upgrades stay with you once you buy them

 - to buy something click on the icon, then click 'Buy', if done correctly the picture
   in the main window will disappear and the item will gain color