imap, doc: documented stuffs on imapsession

I documented some stuffs and now updates the TODO List:

* 
Documentation of each method in IMAPSession.js, parameters and examples, returned data.

* **appendMessage** flags and date parameters should be made more nodeJS friendly + they should be made optional through an option parameter.
* **fetchMessagesByNumber / fetchMessagesByUid** option parameter should also be made more nodeJS friendly.
* 
remove options parameter of **copyMessages**: use only UID.

* **add tests**(what i added)

LGTY?
the `custom-header` and `X-` prefix seems act a same actor? i'm not sure if there is a custom header that doesn't contain a `X-` prefix?