Add a flag to obey NonSerializedAttribute.

This adds PocoJsonSerializerStrategy.ObeyNonSerialized.  If this is
set to true, the serializer will obey System.NonSerializedAttribute
if found on a field or property, and will skip that member entirely.

ObeyNonSerialized defaults to false, preserving the existing behavior.
Merge branch 'obey-nonserialized'