Some nice improvements

Added a nicer lives left indicator with small balls, added a function to
draw a scaled image at a certain position . Added a delay to the
endscreen so that one cannot continue to a new game immediately (and
thus miss the grade). Added a few more reviews and fixed a couple of
spelling errors in the reviews as well as removing one non-PC review.
Added some comments to the gameScreen class.
Added final tennis ball features

Added colissions with walls and other balls. Also fixed the weightFactor
in colissions in general (though that only matters with different weight
balls). I think I still need to tweak some variables to make it feel
"right". Maybe make the variables more dynamic?
Minor changes, before a larger refractoring

Changed the time unit to miliseconds instead of centiseconds.
Large refactoring and added spin

Moved all functions checking and updating the balls and game state to a
new class called Engine. Added a rotating feature to android graphics
and added a spin and angle variable to the Ball class. Added spin when
colliding with walls and when clicking, not yet added to dragging (since
that isn't working correctly) nor to ball-ball collisions.