Support for private/mentioned flag

Adds private flag for 1-1 messages.
Adds mention flag if contains user's nick.

Necessary for client to be able to filter on notification type, see https://github.com/Neamar/notiflow-client/pull/6
hint: this is perfectly correct, but to avoid the burden of hasOwnProperty in a POJO, you could use `Object.keys(messageData).forEach(...)`. That way you also get a nifty closure that can be quite handy in some situations