Added a timestamp property to the MarathonEvent object

We needed a way to get the date/time when events were generated. Thus, I added a dataTimeString property to the MarathonEvent object. This property is populated with the current date/time (UTC timezone) with ISO8601 formatting. Renamed FrameworkMessageEvent to MesosFrameworkMessageEvent to make it clear that this object is for the scheduler.

EDIT: The property is called ```timestamp```