In class we implemented a power function for x^n where x is a double
and n is an integer. This power function was unable to deal with
negative values for the integer n. 

For homework extra credit, extend this power function to deal with
negative values of n.

For positive n,  x^n is x * x^(n-1).
For n = 0, x^n = 1.
For negative n,  x^n is  1 / (x^(-n)).