Add callback-once behavior to models

[ilyavf]

We do both: server responds to the update request with websocket's callback, and also server sends an updated event:

4218["api/contribution_months::update","589a406ec2bc190011c72a29",{ ... }]
4318[null,{"_id":"589a406ec2bc190011c72a29",...}]

42["api/contribution_months updated",{"_id":"589a406ec2bc190011c72a29",...}]

[frank-dspeed]

i think steal-socket.io should do that? i don't understand please more information