down integrate python opensource to svn


git-svn-id: http://protobuf.googlecode.com/svn@551 630680e5-0e50-0410-840e-4b1c322b438d
add shared_pr.h


git-svn-id: http://protobuf.googlecode.com/svn@552 630680e5-0e50-0410-840e-4b1c322b438d