Explanations and examples for IterT and CoiterT
Clarified documentation for CoiterT. Added ComonadEnv instance.
Lifted instances for Comonads (Store, Traced, Hoist) in the `comonad` package
Generalization from Iter to IterT m.
Added 'cut', 'never' to Iter. Renamed 'toIterT'.

The name liftIter is more informative (as the fact that the code is
inside an 'IterT' monad should be explicit already). Also, it's a
monad homomorphism, similar to many things that are named 'lift…'.
Merge pull request #46 from team-free/haddock-iter-coiter

Explanations and examples for IterT and CoiterT