Implement stash/unstash steps for pipeline temporary storage

[rtyler]

These two steps are really helpful in Jenkins and Otto should have something similar.

In order to accomplish this, I think steps may need the ability to register hooks that can run at the end of a stage/context or at the end of a pipeline. Without hooks, the object-store would have to know when a pipeline has exited scope and properly terminate any archived artifacts in the pipeline's "temp" location.