Fix the access to services from non-java language

[cescoffier]

This fixes #14. Long time overdue....

So this PR fixes the polyglot issue of the vert.x service discovery. It's not possible to retrieve the "right" type of service for non-Java callers. It provides new methods when using the service reference approach (low level) and service types.

For example, in JavaScript:

discovery.getRecord(function (rec) {
    return rec.name === "my-http-service"
  }, function (rec, err) {
    var result = {};
    if (! err) {
      var reference = discovery.getReference(rec);
      // reference.getService(Class) is a new method returning the right object
      // HttpClient is the JS HttpClient (got from var HttpClient = require("vertx-js/http_client");)
      var client = reference.getService(HttpClient);
    }
  });

When using service types it does:

HttpEndpoint.getClient(discovery, function (rec) {
    return rec.name === "my-http-service"
  }, function (res, err) {
    if (! err) {
       // res is the right HTTP client
    }
});

In RX Java:

HttpEndpoint.getClient(discovery, record -> 
        record.getName().equalsIgnoreCase("my-http-service"),
        ar -> {
          if (ar.failed()) {
            message.reply("FAIL - no service");
          } else {
            HttpClient client = ar.result(); // Already the right type of object
            message.reply(result);
          }
});

It also works for service proxies, where you need to pass the class you expect to get:

EventBusService.getServiceProxy(discovery,
        record -> record.getName().equalsIgnoreCase("my-service"),
        HelloService.class, // <---- the RX Hello Service class
        ar -> {
          if (ar.failed()) {
            message.reply("FAIL - no service");
          } else {
            HelloService client = ar.result();
          }
});