Fix scoped scenes

ScopedScenes weren't very easy to work with. Now they're much more straightforward.

Thie PR changes the Screenplay lifecyle so that Scene.setUp and Scene.tearDown calls happen before the rigger attach/detach methods are called. This ensures that the ScopedScene can create the Scope in setUp and inject its dependencies before rigging occurs.

It also changes all references of nextScene/previousScene to incomingScene/outgoingScene, for clarity.

An example of a ScopedScene being used has been added to the mortar-sample project.