I added UFO capability to the game. Now, when a certain level is reached (specified in the constants class), a UFO will be spawned.

A few things to note about the UFO:

1) A UFO is spawned to add another level of difficulty to the game.
2) A UFO will shoot at the player entity. If the player entity does not move it wll be hit by the UFO shots.
3) UFO shots will only do one unit of damage to the player entity to either its shields or health.
4) A shot from a UFO will not reduce the firepower of the player entity.
5) If the player entity collides with a UFO while shields are engaged, it will do a defined amount of damage to the player's shields (currently set to 4 units of damage).
6) If the player entity collides with a UFO whle the shields are not engaged, double damage will be done to the players health.