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.
Can you reduce the number of `return`s here by storing `result =`? `pylint` complains about the number of returns.

I think it's OK here, but in other cases it'd be a good warning, so since we don't want to `# pylint: disable=` we should just abide by it here.