Scotty Monad -> Scotty Monad Transformer

Hello.

This pull request changes the underlying Scotty typesystem from the Scotty monad `ScottyM` to a Scotty monad transformer `ScottyT`. I think this is an important and useful change. While I like using Scotty its DSL is very small and limited - and that's OK, it serves its purpose - however the problem is that its DSL is not extensible. By switching to Monad transformers we allow users to create customs DSLs on top. I needed to implement this change in order to make scotty-hastache: <https://github.com/co-dan/scotty-hastache>, <https://gist.github.com/co-dan/6015894>

What do you think about this?

PS:
In this version `ScottyM = ScottyT IO` and all the user code still works flawlessly, this does not break API.