Major graphics update

 * Changed all existing graphics (missile, stickman,
   background, explosion, logo) to new ones.
 * Added animation sequence for explosion instead of the
   previous single image being expanded.
 * Added images and background in splash screen.

 * variable 'x' in GameCharacter.java now denotes
   position of center of stickman. This allows
   running_stickman and standing_stickman to be of
   different width.