[commands] Fix for ```send_video``` failing without a caption string given.

[json] Add user online status event

(see https://github.com/vysheng/tg/issues/538)

E.g. ```{"event": "online-status", "online": true, "when": "2015-05-20 00:16:40", "state": 1, "user": {"username": ... }}```

E.g. ```{"event": "online-status", "online": true, "when": "2015-05-20 00:16:40", "state": 1}```

Added output when message are marked read (if ```--json```), which fixes Issue [#516] (https://github.com/vysheng/tg/issues/516#issuecomment-101710890)
Also added event field to json which will be send to terminal/main_session:
"event" = "message", "read" (message got read), "service", (events only in secret chats?)  "updates" (peer name change etc), "download" (file downloaded)

Added username field to message table passed to Lua

Added username field to message table passed to Lua