Fixed trailing whitespace.

Stupid GitHub editor introduced trailing whitespace.
Merge pull request #58 from merijn/patch-1

Fixed trailing whitespace.
Added 'ListT' section to the tutorial
Merge branch 'master' of github.com:Gabriel439/Haskell-Pipes-Library
Generalized `Consumer`s in Pipes.Prelude to `Consumer'`s
Generalized `foldl` and `foldM`

This is so that they will work with the upcoming `foldl` package