Provide a way to know if a message was delivered/read

## Description
Provide a way to know if a message was delivered or read *without listening all the time*.
Basically add read_by and delivered_to attributes to Message object.
At least fetchThreadMessages() should fill those attributes.
read_by and delivered_to could be dict where key is user id and value is the timestamp when the message was delivered/read (should be possible to find at least for read time because FB show it in browser).

## Researches
Looks handled in js by FB (and obfuscated)

Any help to understand how it works is welcome ;)
This issue allows us to follow the progress of our searches.

BTW, is_read attribute (in Message object) looks confusing, should be better to rename it to read_by_me or something like that, no?