#NO_ISSUE Constraints refactoring
Merge pull request #253 from drexin/wip-refactor-constraints-drexin

#NO_ISSUE Constraints refactoring
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.