Add read receipt message type

[devchakraborty]

I have no idea why this started happening, but Facebook started sending me messaging payloads that look like this:

{"sender"=>{...}, "recipient"=>{...}, "timestamp"=>1465354035944, "read"=>{"watermark"=>1465354034142, "seq"=>33}}

which raises an UnknownPayload and breaks my app 😱. This behaviour doesn't appear to be documented by Facebook yet.

This PR adds support for this new type of payload.

[coveralls]

Coverage decreased (-1.3%) to 98.272% when pulling 5e8e145 on devchakraborty:master into bfff15c on hyperoslo:master.

[coveralls]

Coverage decreased (-0.8%) to 98.704% when pulling 3bf69b2 on devchakraborty:master into bfff15c on hyperoslo:master.

[mengqing]

I was wondering whether Facebook::Messenger::Incoming::UnknowPayload should be raised at all if the incoming payload can not be recognized rather than just ignore the payload. This is so that in case Facebook decides to add new payload in the future and every time they do that, the app would be broken

[jgorset]

Awesome, @devchakraborty! This is why we <3 open source. Thank you.

[jgorset]

I agree, @mengqing – let's log a warning, not crash with an exception?

[jgorset]

This might just start happening to everyone, so I released version 0.6.0.