add: EnumPassthru on ProtoContractAttribute

git-svn-id: http://protobuf-net.googlecode.com/svn/trunk@648 b72047a4-3652-0410-9617-b3994939e97b
FIX: Issue307

git-svn-id: http://protobuf-net.googlecode.com/svn/trunk@649 b72047a4-3652-0410-9617-b3994939e97b