ForceGameOver; game over screen. Fixes #29

You can now force the game to end from the pause screen. This is different from the quit button:
- it only shows up in debug builds. "build settings > development build", I think
- it ends the game as if time has run out, or whatever our gameover condition's going to be

This is all so we can actually see the game over screen.
Gameover sound effect. #29