ProtoReader recycling facility

git-svn-id: http://protobuf-net.googlecode.com/svn/trunk@659 b72047a4-3652-0410-9617-b3994939e97b
Adds a FAILING TEST - the last commit has a regression; will fix later today

git-svn-id: http://protobuf-net.googlecode.com/svn/trunk@660 b72047a4-3652-0410-9617-b3994939e97b