Added regression tests for 10 updates. Cleaned up static methods.
Added UpdatableItem, unit tests, changed two lines of the original code
to use the new Item's update() methods.
Added Legendary Item type that does not degrade quality. Replaced
"Sulfuras" with this type.
Added minimum quality of zero to UpdatableItem
Added Cheese Item type that increases in quality, modified main loop to
not exclude Brie from the basic update()