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
If the archive is not generated, there's no issue. 

if unboundJarName is set, it will be likely done after applying the plugin in the build.gradle, something like that :
````
apply plugin: 'jibx'

.....


JIBXBinding {
    rootAPIPath = 'my/path/to/bind'
    bindingFiles = ['./path/binding.xml']
    unboundJarName = 'my-unbound-jar.jar'
}
````

The closure configuring the 'generateUnboundJar' task will be executed when the plugin is applied, unboundJarName won't be set yet and so the archiveName won't be too.

The onlyIf part being still in the afterEvaluate block will enable the archive generation since unboundJarName is set at this time but I fear the unbound archive won't have the expected name.

It can be checked easily with this simple gradle build script

````
apply plugin : 'jibx'

task check {
    println "unboundJarName value : $JIBXBinding.unboundJarName"
}

JIBXBinding {
    unboundJarName = 'testvalue.jar'
}
````

calling gradle check won't do anything but during the initialization phase you'll have the following statement :

unboundJarName value : null