Implemented a method to set the speed of the creature. This method by
default sets the speed to 1 (Essentially adding 1 every timer interval.
Subclasses can call this method to set their speed higher or lower
(Though since speed is an integer it can't go lower than 1. (Maybe
should work in some error throwing/catching if it is given a value lower
than 1)).
Added the prey class and now when the game starts it has prey that move
independently of the predator.
Added the fast and slow prey classes and changed the method in
AliensvsPredator class to add fast and slow prey with random amounts of
each.