Add support of the certchain parameter to the "sign" mojo

"certchain" parameter has been added to jarsigner in Java SE 7. In my case it is required to do signing of JAR's with a hardware key (see JENKINS-37567).

I propose to add a new option to JAR Signer Lib and to Maven JarSigner Plugin. This option should be opt-in in order to keep the plugin compatible with the Java 6 flow