Added Lives and GameOver

Lives class added
-displays lives as integer below score
-life count initially set in Game.cs to 3
-bool AddToLives(int num)  adds num lives to life count and returns
false if the life count has dropped below 1
-to decrease life count and check for game over, add code like
if (!lives.AddToLives(-1))
game_over.gameOver = true;

GameOver class added
-simple class that basically holds boolean value for whether or not game
over has occurred and handles displaying game over message
-extra functionality could be added
-GameOver class used in Game.cs to disable all button presses other than
Esc after a game over has been reached