This pull request addresses issue #60.
The change updates MappingSchema to watch for a new colander.drop value during deserialization. If a node deserializes to that value, it is excluded from the schema's deserialized output.
specify optional fields with "missing=drop"
This pull request looks good to me, and it seems from some previous dialogue that it is an acceptable feature request. It just needs docs. I can do this in a separate branch, and submit a pull request.