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.
(this convention was put into place before nix supported dashes in identifiers. Hopefully some day it will change)