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 2014-12-20 01 17 43](https://cloud.githubusercontent.com/assets/111265/5512605/ac954cf4-87e6-11e4-8782-6128d701d0e7.png)