Update to 0MQ4 + cleanup

The examples now work with the latest version of 0MQ, using [zeromq4-haskell] (https://hackage.haskell.org/package/zeromq4-haskell).

* All written examples have been checked.
* Comment, description have been added.
* A cabal file has been added to ease setup (some examples require the `-threaded` compilation flag and this is now explicit.
* HLint cleanup