The fibonacci sequence is a famous bit of mathematics. The first two values in the sequence are 0 and 1. Each subsequent value is the sum of the previous two values, so the whole sequence is: 0, 1, 1, 2, 3, 5, 8, 13, 21 and so on. Define a fibonacci(n) method that returns the nth fibonacci number, with n=0 representing the start of the sequence.