Documentation and references for Applicative, Cofree, Free, and Free.Church 

Somewhat more detailed documentation for several modules in the package. In particular, it includes links to two papers by Neil Ghani that give a motivational background for the monadic instances of `Cofree`.

Together with pull requests [44](https://github.com/ekmett/free/pull/44) and [46](https://github.com/ekmett/free/pull/46), this gives the warm, fuzzy feeling of 100% haddock coverage.