Fix duplicate watchers from being added

[sudowork]

Based on the existing code for WatcherManager.js, it seems like adding a duplicate watcher should not be allowed. However, the current check for watchers[path].listeners('notification').indexOf(watcher) === -1 will never be false. This is because EventEmitter ends up wrapping the listener before adding it if once() was used.

Here's a contrived test based on the examples/get.js example:

var zookeeper = require('./index.js');
var client = zookeeper.createClient(process.argv[2], { retries : 2 });
var path = process.argv[3];

function watcher(event) {
    console.log('Fire me once: %s', event);
    getData(client, path);
}

function getData(client, path) {
    client.getData(
        path,
        watcher,
        function (error, data, stat) {
            if (error) {
                console.log('Error occurred when getting data: %s.', error);
                return;
            }

            console.log(
                'Node: %s has data: %s, version: %d',
                path,
                data ? data.toString() : undefined,
                stat.version
            );
        }
    );
}

client.once('connected', function () {
    console.log('Connected to ZooKeeper.');
    getData(client, path);
    // @NOTE: this should not register a second watcher
    getData(client, path);
});

client.connect();

Output before the patch:

$ node test.js localhost:2181 /test
Connected to ZooKeeper.
Node: /test has data: hi, version: 5
Node: /test has data: hi, version: 5
Fire me once: NODE_DATA_CHANGED[3]@/test
Fire me once: NODE_DATA_CHANGED[3]@/test
Node: /test has data: hi, version: 6
Node: /test has data: hi, version: 6

Output after the patch:

$ node test.js localhost:2181 /test
Connected to ZooKeeper.
Node: /test has data: hi, version: 6
Node: /test has data: hi, version: 6
Fire me once: NODE_DATA_CHANGED[3]@/test
Node: /test has data: hi, version: 7

[alexguan]

Thanks for the PR, I will take a look this weekend and merge it in manually since I need to bump the version for the fix.