Add MonadBase and MonadBaseControl instances for FreeT

Because I know how much @ekmett just *loves* monad-control... but seriously, I found myself needing these and thought they could be of general utility to others.  My use case at the moment is a concurrency Applicative built on FreeT, in the spirit of Marlow's HaXL monad: https://gist.github.com/6905323