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