Support promise

[h2jorm]

This is a nice zookeeper library. However, compared to promise, the callback style is not coding friendly today. Some issues or pull requests are related to promise API. For example, #47 #58 #87 .

I have made the following methods promise supported.

• Client.create
• Client.remove
• Client.exists
• Client.getChildren
• Client.getData
• Client.setData
• Client.getACL
• Client.setACL
• Client.mkdirp
• Transaction.commit

Example

zookeeper.create('/a', Buffer.from('hello'))
    .then((path) => {
        assert(path === '/a');
    }, (err) => {
        console.log(err);
    });

This PR does not add new methods and backward compatible. In general, the methods above will return a promise if they fail to receive a callback function as the last parameter.