Added test and logic for the requirement that item quality never exceeds 50
Rename method decreaseSellByDate() to decreaseSellInDays()
Extracted new BackstagePassItem class from UpdateableItem, with tests; modified Runner to use new class
Removed extracted logic form UpdateableItem
Fixing spacing
Extracted method from UpdateableItem to SulfurasItem; added test
Added test for requirement that standard items decrease in quality by two after sell by date is passed
Replacing some integer constants with explanatory methods
Renamed UpdateableItem to StandardItem