added monad implementation to (Free f a) datatype: excersice 3 (Monads)

Hey Paul.
Im currently on the path to enlightenment in Haskell, using the typeclassopedia. I have had a lot of use of your solutions, and I want to thank you for putting them up on github. I saw that there was a missing implementation on the monad chapter so I took the liberty to add a small modification.