Add comments. Solution 4: Largest palindrome product
Solution 5: Largest palindrome
Added asserts + clean-up
Solution6:
Solution7: 10001st prime
Solution8: Largest product in a series
Solution 9: Special Pythagorean triplet
Solution10:
Solution11: Largest product in a grid
Added test for multiply extension
refactoring, code clean-up
Solution12: Highly divisible triangle number