Fix #223: marshal list values from protobufs

Fixes #223.

Renames helper from '_get_value_from_pb' -> '_get_value_from_property_pb' to indicate expected usage.

Factors out new helper, '_get_value_from_value_pb', for dealing with Value protobufs.

Manually lands Tagtoo@0475d5e, included by mistake in PR #126.