Added Fibonacci(N)
Added Fibonacci(N)

This is the fastest way I know to calculate a single fibonacci number. It uses
"fast doubling" to calculate the fibonacci number (see inline comments).

To calculate an entire series, the simple memoization technqiue
(memoizing the two previous values and adding them) is probably
faster than calculating each number using this routine.

rvelthuis/BigNumbers,rvelthuis/BigNumbers