Bring recent Haskell tools

When you start working with Haskell - developing or installing packages from source - the first step is to install the platform. The two required tools (which can be used to install all the others) are GHC and cabal-install.

Debian has packages for them both, but their versions are getting old. Developers use more recent versions, which usually means they download from external, such as PPAs or the Haskell Platform. Ubun7u inherits these from Debian, and Trisquel inherits them from Ubun7u.

This merge request brings the recent stable recommended versions of GHC and cabal-install from a PPA, so people can start with Haskell more easily without being confused by all the different external download locations.

NOTE ON HOW IT WORKS: In the PPA, there are several GHC versions and each one is built using the previous one, so there is a chain of build-depends between them. I solved the bootstraping problem by having this new GHC 7.8.3 be built using the existing GHC package coming from debian/ubun7u. Jenkinds successfully built it.

• The debian package is GHC 7.6.3-10 + some patches
• The PPA package used for building the new GHC 7.8.3 is 7.6.3-9 (without debian's patches)

Since they're nearly identical (I did a diff -r to see all the differences), I gave it a try. I was lucky and GHC 7.8.3 indeed builds with debian's GHC 7.6.3.