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 :)
Oh ... you made -legacy.js this part in libetpan.
Actually, I think that what is in imap-legacy.js would have made simpler this implementation.
Could you reconsider adding imap-legacy.js back?