Added Problem 8 Solution

Added the solution for problem 8 (Largest product in a series). Also did
some cleanup of Problem7's Haskell solution - It was a complete mess!
Unfortunately I coded it quite some time ago, and I don't really
remember the particulars... so even though it is better than before it
still looks like crap.

This is why we write good, readable, commented code the first time
around people!