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
I got it but it seems (in gradle doc) that plugin Extension are resolves (or mapped) before.
So if you write this (example come from doc):

    apply plugin: GreetingPlugin
    
    greeting {
        message = 'Hi'
        greeter = 'Gradle'
    }
    
    class GreetingPlugin implements Plugin<Project> {
        void apply(Project project) {
            project.extensions.create("greeting", GreetingPluginExtension)
            project.task('hello') << {
                println "${project.greeting.message} from ${project.greeting.greeter}"
            }
        }
    }
    
    class GreetingPluginExtension {
        String message
        String greeter
    }

The result should be:
    > gradle -q hello
    Hi from Gradle
http://www.gradle.org/docs/current/userguide/custom_plugins.html (59.3)
About our plugin, I have done some tests and it works fine.