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?
There are custom header that contain a X-Prefix. And some headers are not listed as part of the "headers" or "full-headers".