Initial implementation of ContT

Based off of:
https://gist.github.com/paf31/2dfcefe275db262beb42
Merge pull request #1 from ethul/topic/continuation-monad-transformer

Initial implementation of ContT
Add Applicative and Functor instances for MaybeT
Adding MonadCont instances
Merge branch 'topic/monad-cont-instances'
Update classes for 0.5.0
Fix <*> for StateT
Fix pure for MaybeT
Update ignored files
Add execWriter
Update build
Add generated docs