Added support for NSUUID serialization & Custom Date Formatters

When using an OData query we need to know if a value is a GUID because they have to be formatted differently.  So we can't just serialize a GUID as a string as we normally would.

Also I've had the need in several projects to use a custom Date Format in the serializer.  This is at least the 3rd time I've needed it so it breaks the barrier to get added to core.