Update HandlebarsEngine.java

Handlebars layout now implemented. Usage (from groovy for example)
Have a file named layout.hdb that contains {{TemplateBody}} and a file bodyOnly.hdb which contains the main template body
...
new GYoke(vertx, container)
  .engine('html', new GroovyTemplateEngine())
  .engine('hdb', new HandlebarsEngine()).
  .use(new GRouter()
	.get("/handleBarsTemplateWithLayout") { request, next ->
		request.put('session', [id: '1'])
		request.response.render 'bodyOnly.hdb','layout.hdb', next
	}
	)
  .use {request ->
    request.response.end "This is the default route: maybe you should go to /handleBarsTemplateWithLayout"
  }.listen(8080)