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 :)
Could you rename this file to imapsession.js?
imap.js will probably include imapsession.js and imapmessage.js