Make changes to make Riff-Raff aware of 'stack' and 'app' concepts

 - Add new 'stack' field to deploy.json
 - Rework App class into two classes for both legacy and stack-app concepts
 - Enhance the Lookup classes to handle lookups using both App subtypes
 - Show stack in the deployment information screens
Add some documentation changes for stack and app.
Fix up the tests in light of changes to App classes
Re-implement lookup stub so that both App types can work (no longer use DeployInfo shim)
Merge pull request #143 from guardian/sh-stack-and-app

Add concept of stack and app to Riff-Raff