Add a Config class to make configuration understandable

This is an attempt to unify the jank in summingbird-scalding, internal to Twitter, and in the Job code.

Next up, a clean way to start and run jobs without a Job class.
This method should only be called once (it overwrites the keys it touches). Generally, users would not call this (even the Config.default sets up something reasonable).

I thought about that approach too, which I like more in many ways. A better approach might be to build up a Function from `Config => Config`. Then, before we run, we put in the external configuration (from the JobConf, or Args, or whatever), and the function applies to that.

This just seemed simpler at the time. Feel free to improve it.