add text handler

To add a route using TextHanler : 

  get(new Route<Void, String>("/nextCommand", Void.class, MediaType.ANY_TEXT_TYPE) {
            @Override
            public Response<String> handle(Void param, RouteParameters routeParams) {
                String next = elevator.next();
                LOGGER.info("nextCommand = {}", next);
                return new Response(next);
            }
        });


default using JsonHandler for retrocompatiblilty