Move createSerializedForm to DS.Serializer.  Serializer calls the routine currently, but it's only defined in the JSON and Fixture implementations.
Fix belongsTo relationships to models with id 0
Merge pull request #876 from tricknotes/fix-belongs-to-relationships

Fix belongsTo relationships to models with id 0
Remove unused local variable
Merge pull request #877 from tricknotes/remove-unused

Remove unused local variable
Merge pull request #812 from zachriggle/master

Move createSerializedForm to DS.Serializer