The Fibonacci sequence is a somewhat fundamental mathematical/natural
sequence whose core property is that every Fibonacci number is equal to the sum
of the previous two numbers.
More formally,
Fib(n) = Fib(n1) + Fib(n2) and
Fib(0) = Fib(1) = 1 .
This results in a sequence like 1, 1, 2, 3, 5, 8, 13, 21...
Write a function which takes one parameter, an integer "n" that
returns the nth number in the Fibonacci sequence.
