New game mode

With this release, you can play Powerless in 3 game modes:

• Purge: No time limit, kill a certain number of zombies to win the game
• Survival: Free play, survive as long as you can as zombies constantly spawn
• *New Mode* Gauntlet: Zombies spawn in waves, defeat each wave before advancing to the next.  Defeat all 30 waves to win the game.