Added outTime plug to SceneShapeInterface that is directly computed from the input time plug. Used to connect input time of children to parent outTime when expanding.
Merge pull request #24 from valeriebernard/sceneShapeOutTimePlug

Added outTime plug to SceneShapeInterface that is directly computed from...
Added more tag testing in IECore LinkedSceneTest which shows a bug in readTags/hasTag.