[REEF-33] Allow Tasks to initiate an Evaluator Heartbeat

  This adds the class HeartBeatTriggerManager which can be used by a Task to
  initiate an Evaluator Heartbeat. It is used by injecting it into the Task.

JIRA:
  [REEF-33] https://issues.apache.org/jira/browse/REEF-33

Pull Request:
  Closes #24