Rename helper memor to indicate usage.

Expected argument is a Property protobuf.
Factor out value_pb bits from '_get_value_from_property_pb'.
Add support for 'list_value' field of a Value protobuf.

Fixes #223.

Manually lands Tagtoo@0475d5e, included by mistake in PR #126.
Kowtow to pylint's over-fussy 'too-many-return-statements'.

Incorporates feedback from @dhermes.
Merge pull request #246 from tseaver/223-marshal_list_values

Fix #223: marshal list values from protobufs