rnd-20131212-1-src

* removed limitation of artwork files to selected file types (this means
  that every file type supported by SDL_image and SDL_mixer can be used)
* changed default graphics vom PCX to PNG (needed for Android version to
  prevent painfully slow loading of images, although not compressing PCX
  files in the assets directory of the APK package might also work fine)
* fixed bug with SDL_BlitSurface creating garbage when source and target
  surface are the same (this bug also existed in versions of SDL 1.2.x)
* started porting Rocks'n'Diamonds to Android