Proposal for Shortstack v2

[Prettified README](https://github.com/ReturnPath/shortstack/tree/rm/proposal)

We've had a lot of time to explore the current implementation's strengths & weaknesses, and I think there's _some_ consensus on what shortstack's boundaries should be.  Now that we're moving towards using Docker for managing execution environments, it seems like there's an opportunity to radically simplify what 'Shortstack' does.  

This is an idea based on some discussions I've had with Peter, Ben and a few others.

This is being submitted to the RP repo rather than the OIB one so if we agree on the direction we can start from scratch and Open Source it from the start (without worrying about what credentials are in the git history)