Expose events for server backoff

[n1k0]

Refs #68, #82. We should expose public events for server backoff, so users can subscribe to them to handle UI changes accordingly, eg:

kinto.on("server-backoff", seconds => {
  if (seconds > 0) 
    $("#sync-btn").disable();
  else 
    $("#sync-btn").enable();
});

[n1k0]

Fixed by #88.