Add zeromq4-haskell.

In zeromq-haskell/issues/58 I have been asked to consider adding
zeromq4-haskell to stackage. The package builds with recent versions of
its dependencies from hackage, but I did not test against stackage.

Please note that--being an FFI binding--this package depends on zeromq
>= 4.0.
Merge pull request #301 from twittner/zeromq4-haskell

Add zeromq4-haskell.