merge tags/2.6.0 into trunk


git-svn-id: http://protobuf.googlecode.com/svn@601 630680e5-0e50-0410-840e-4b1c322b438d
change back to gtest head


git-svn-id: http://protobuf.googlecode.com/svn@602 630680e5-0e50-0410-840e-4b1c322b438d