add the servant (haskell) packages to nixpkgs

This adds the set of servant Haskell packages to nixpkgs.

- [servant](http://hackage.haskell.org/package/servant)
- [servant-pool](http://hackage.haskell.org/package/servant-pool)
- [servant-response](http://hackage.haskell.org/package/servant-response)
- [servant-postgresql](http://hackage.haskell.org/package/servant-postgresql)
- [servant-scotty](http://hackage.haskell.org/package/servant-scotty)

(nix expressions generated with cabal2nix)

Let me know if there's something wrong in there.