support additional attributes for some objects

[smcoll]

i was going to leave a ticket about adding support for Signer.excludedDocuments, but realized that there are actually several attributes not supported by that object. It may be cumbersome to explicitly include all sorts of untested attributes, so perhaps a compromise would be to allow the passage of additional keyword arguments into the objects on instantiation. Whatever solution would have to also be implemented in the to_dict method.