Problem #10 - finding the summation of primes under n. Added two different solutions using Ruby. Also refactored out some code for later reuse.
Added a solution to problem #11 - finding the largest product (of 4 numbers) in a containing grid. Also cleaned up comments.
Added solution to problem #12 - highly divisible triangular numbers.
Added solution to problem #13 - summing large numbers.
Added solution to problem #14 using memoization and recursion.