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
Indeed but archiveName won't be correctly set :

````
+        def generateUnboundJar = project.task(type: Jar, dependsOn: 'JIBXResources', 'generateUnboundJar') {
+            classifier = 'nojibxbinding'
+            if (project.JIBXBinding.unboundJarName !=null) {
+                archiveName = project.JIBXBinding.unboundJarName
+            }
+            from project.sourceSets.main.output.files
+        }
````