Added a parameter to the MockSprite constructor

The additional parameter lets you choose the starting position of the sprite.
Added a class that represents the screen
Fixed incorrect unittest

Also fixed errors (due to buggy unittest) in class Screen
Added updated memory to Screen

This will enable dirty rect updating if the frontimage attribute of a Screen
instance will be chosen as the physical screen.
Wrote a better test for Screen.updated

First of all it also tests moving sprites.
Secondly it also accepts any order for the elements in the updated list, as
long as it contains every dirty Rect exactly once.
And finally it contains the dirty Rects for the cleared and the drawn areas
instead of just the cleared areas.