Creating a Fleet creates a GameServerSet

All this commit is doing is create a GameServerSet
when a Fleet is created. Also, if the Replicas in
a Fleet are updated, then the owned GameServerSet's
Replicas are also updated.

For this commit, we are not worrying about handling
updates to the Spec (e.g. new image) to the Fleet,
and doing updates to a live Fleet, we are just concerned
with scaling up and down.

Parent ticket: #70