Update Haskell-related formulae

Hey, I updated some Haskell-related formulae to the newest versions now that Haskell Platform finally got updated:

* GHC 7.6.3
* cabal-install 1.16.0.2
* Haskell Platform 2013.2.0.0

I cleaned up the GHC formula quite a bit and transitioned it back to working with the official bindist packages, based on @camillol's work in #18779.

Tested on OS X 10.8.3. More testing welcome!