Added a background image to all opening menus; Fixed health bars
sometimes showing underneath boss sprites; Accidentally found a neat
new graphical effect while messing with the pause screen's background
and decided to leave it in (it actually looks quite nice).
Merge branch 'master' of https://github.com/joster422/KnightsVsZombies.git
*Sigh* Something weird just happened with GitHub again.
If this commit pushes correctly (which I don't think it will):
Added a background for all opening menu screens; 
Fixed health bars not showing correctly for bosses;
Added a new graphical effect when pausing the game