P21: Adding the problem statement.
P21: First attempt, just threw the code in there.  It found 3 pairs when going up to 10000 but apparently it wasn't right.
P21: Had a small error in my pair summing line that needed to be corrected.  But the answer is still wrong.
P21: Finished problem.  Was only checking first half of MAX for pairs.  Needed to check the second half of the range.
Merge branch 'problem21'