Add some faces for structured-haskell-mode

This change adds support for block highlighting using [structured-haskell-mode](https://github.com/chrisdone/structured-haskell-mode).

![screenshot 2014-12-22 14 25 06](https://cloud.githubusercontent.com/assets/111265/5524951/ff8cde2a-89e6-11e4-9948-7f2a3b6a9c6b.png)