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
Hum, thinking back on this -- you were right. Keeping undefined is better, it won't be sent by GCM and we'll get smaller messages. Let's keep it that way, and maybe update other booleans flag to use undefined too?