improve configurability and make redis optional

This is a quick proof of concept that depends on unreleased code, but could be merged following release of next Derby & Racer alphas if the addition of [convict](https://github.com/mozilla/node-convict) doesn't bother anyone.

The tarballs in package.json are currently tags of derby & racer master. They're there because c9.io can't resolve git locations right now.

Environment defaults proposed do not represent strong opinions, but just examples.