Added cask for SuperTuxKart

Refactored SuperTuxKart cask

- changed file name from super-tux-kart.rb to supertuxkart.rb
- changed class name from SuperTuxKart to Supertuxkart
- changed sourceforge url to point directly to actual Mac app
Merge pull request #4744 from poltak/supertuxkart

Added cask for SuperTuxKart