[Factorial - Wikipedia]((http://en.wikipedia.org/wiki/Factorial)
From Wikipedia:
In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n.
For example, the factorial of 5 is 120 or (5 * 4 * 3 * 2 * 1).
Write a ruby program that calculates the factorial of a given number.
factorial(3)
=> 6
factorial(4)
=> 24
factorial(5)
=> 120