Add jar command (*** Work in progress ***)

Add a jar command that produces a self-contained executable JAR file
for a CLI application.

A lot of polish is required, but the new command is now functional
for a basic scenario at least. Running spring jar app.groovy will
produce a jar named cliapp.jar in the current working directory. This
jar is self-contained, i.e. it contains everything needed to run the
application. The application can be run with java -jar cliapp.jar.

The command has been tested with the simple web sample:

@Controller
class Example {

        @Autowired
        private MyService myService;

        @RequestMapping("/")
        @ResponseBody
        public String helloWorld() {
                return myService.sayWorld();
        }

}

@Service
class MyService {

        public String sayWorld() {
                return "World!";
        }
}