Add example code for ACL

[DavidVujic]

Is your feature request related to a problem? Please describe.
There should be a working runnable code example for using the ACL feature of the library. Example code exist in the README, but that one is not longer valid (the API was probably different at the time of writing). The current example will be removed from the README shortly.

Describe the solution you'd like
A JavaScript file with ACL example code, in the examples folder. The code should use the Promise based ZooKeeper client. This file should be able to run using node examples/acl.js.

Additional context
This is the example code (that is not currently working) from the README:

var ZooKeeper = require("zookeeper");

zk = new ZooKeeper({
    connect: "localhost:2181",
    timeout: 2000,
});

var key = "/acl-test";

zk.connect(function (err, client) {
    if (err) throw err;
    console.log("zoolocker: Connected to Zookeeper, id=%s", zk.client_id);

    client.add_auth("digest", "username:password", function (rc, error) {
        console.log("ADD_AUTH", rc, error);

        client.a_create(key, "", {
            version: -1
        }, function (rc, error, path) {
            console.log("CREATE", rc, error);

            client.a_set_acl(key, -1, [ZooKeeper.ZOO_CREATOR_ALL_ACL, ZooKeeper.ZOO_OPEN_ACL_UNSAFE, {
                perms: ZooKeeper.ZOO_PERM_WRITE,
                scheme: "world",
                auth: "anyone",
            }], function (rc, error) {
                console.log("SET_ACL", rc, error);

                client.a_get_acl(key, function (rc, error, acl, stat) {
                    console.log("GET_ACL", rc, error, acl);
                });
            });
        });
    });
});

[nitin-pandita]

I would like to work on it