Add two new system notifications, for clock skew and missing master in cluster

also refactor the notification class slightly to guard against missing timestamp and node id

fixes Graylog2/graylog2-server#401
fixes Graylog2/graylog2-server#402
fixes Graylog2/graylog2-server#403
Merge git://github.com/Graylog2/graylog2-web-interface into develop