Proofs for Applicative, Monad, MonadTrans instances of CofreeT.

In response to [comment in issue 41](https://github.com/ekmett/free/pull/41#issuecomment-36176010).

Includes proof of all laws for each instance, and consistency between the Monad and the Applicative implementations. 

GitHub-flavoured markdown syntax is used for formatting, which is also readable as plain text. The files follow the same directory structure as the sources they refer to. They are also included by cabal in source tarballs.