Header:Added a Moderator String field to the PlanningPokerGame model. Also added a Serializer for it.

Reported-by: lhnguyenduc
Signed-off by: Long Nguyen Duc Hoang <lhnguyenduc@wpi.edu>
Merge pull request #119 from hlong290494/sbl-dev

Moderator field for the PlanningPokerGame model