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?
Here's the other way to document callback:
```
callback: function() { /* ... */ }
```

Maybe you could have a look at this page:
https://developer.chrome.com/apps/app_window#method-create
It gives a way to document arguments of functions.