Optimised Python solution to Problem 5.

This is still a brute force approach, but a lot faster. Optimised by iterating
over multiples of the highest divisor, instead of all natural numbers, d'oh.
Also checking for even divisibility in reverse divisor order and removing 1
from divisors, D'OH^2... (although I don't think the latter has any practically
measurable impact on performance in current implementation).