Very Basic PR

Hi, @dinhviethoa,
I just wrote some basic stuffs(most on structure) and could you please review my source codes, if i make some errors, let me know.

Otherwise, this PR also is a complete PR but within some TODO for us, So you could run the `example.js` to check the functionality of mine.

Also /cc @MailCore's other members :)
You could see my [example case](https://github.com/yorkie/mailcore.node/blob/master/examples/simple-imap-fetch.js), and for implementation(us) we should emit the `error` function once there are some problems, and the function `connect()` also own a `callback` function, but it more like a shortcut for the following:
```js
var imapsession = new IMAPSession(/* arguments */);
imapsession.connect();
imapsession.once(function connectCallbackShortcut() {
  // TODO
});
```