the referenced_value project

This is the implementation of the resource branch outlined in my blog posting http://www.twobraids.com/2013/11/configuration-is-eating-my-brain.html  I would really like to see this system integrated into configman.  We may choose not to use it, just like we're not using the migration feature.   Since configman is a system that is not used outside of Socorro, except for a few personal projects, I think we can afford to be exploratory: we should try different things.  This may prove to be very useful, or, like the migration feature, may fall flat on its face.  We can always back it out later if gets in the way.