Define get_divisors and is_abundant functions
Problem 23 - Non-abundant sums - THE POWER OF THE HASH

* Store all abundant numbers in an array and store all it's indexes in a hash - actually... I need just a Set.. gimme a minute..
Problem 23 - Interesting.. using only a set instead of an array AND a Hash is a bit slower... but it's not noticible and way easier to understand, so I'm leaving the set implementation :)