Several changes, some highlights:
* Structure of haskell-packages.nix updated. It's now easier to
select different default versions of packages for different
versions of GHC.
* GHC 7.0.2 is now default.
* Haskell Platform 2011.2.0.0 has been added and is now default.
* Several packages have been updated
(gtk2hs, gitit, xmonad, darcs, ...).
* Some old packages have been removed.
svn path=/nixpkgs/trunk/; revision=26288
