Rename Enemy.prefab to PurpleEnemy.prefab
Duplicate PurpleEnemy into RedEnemy, add red-colour dependencies

* Create vehicle_enemyShip_red_metal_mat.mat material referencing
  vehicle_enemyShip_red_dff.tif
* Don't create/add vehicle_enemyShip_red_glow_mat.mat as it can't
  be added to the prefab due to imported model's limitations
Check if GameOverText != null before ending game

gameController.GameOver() is called form Player's OnDestroy(), but
Player is also destroyed when user quits game without Player actually
dying. This means all objects are destroyed, possibly GameOverText
before Player.

Similar commit checking if GameController != null was done on
Sun Jun 1 23:06:27 2014 +0100.
Spawn red and purple Enemies with equal chance