Fix compatibility with maven artifacts

* make generateUndoundJar task accessible from project config
example : 
artifacts{
	archives generateUnboundTask
}

* add classifier to be able to upload generated jar into a repo
In fact there is no problem. As we have 

    generateUnboundJar.onlyIf {
        project.JIBXBinding.unboundJarName != null
    }

if unboundJarName is null, the call to generateUnboundJar.output will be just skipped.