Finished (for now) credits screen

Credits screen has a back button and automatically adjusts text to
nicely position it. It might need scrolling capabilities but for now
it's fine.
Added SplashScreen and LevelChooserScreen

SplashScreen stays there for 5 seconds, clicking on it makes it go away
sooner.
LevelChooserScreen not finished.
Merge remote-tracking branch 'origin/Fraser'
Merge remote-tracking branch 'origin/master' into michael3