Add "P93 - " for P93 problem description

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
P93 - Provide link to http://www.haskell.org/haskellwiki/99_questions/Solutions/93

Because my code is largely inspired by the solution there.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>