=pro enable cross builds

* changed zeromq-scala-binding library
* added dependency helpers from master by Johannes.
* modified build-release task to include distributions from cross builds
* removed tar-zip step from release script, because tgz package is not used from the website
* release script now releases for all crossScalaVersions
* dry run is the default now

I will port to master some of this.
are we sure that it is binary compatible for 2.10, so that we don't break next akka release?